Pipenv: dependencias incorrectas (versiones) bloqueadas por pip

intenté agregar flower al proyecto, pero pipenv lock está eligiendo la versión más nueva tornado (5.1) en lugar de una especificada en los requisitos (tornado>=4.2.0,<5.0.0 ) (probado en 2.7.15 y 3.7)

Resultado Esperado

versión apropiada del tornado para ser elegido

Pasos para replicar

$ mkdir tmp1
$ cd tmp1
$ pipenv instalar flor

Versión de Pipenv: '2018.7.1'

Ubicación de Pipenv: '/Users/jerzyk/.pyenv/versions/3.7.0/lib/python3.7/site-packages/pipenv'

Ubicación de Python: '/Users/jerzyk/.pyenv/versions/3.7.0/bin/python3.7'

Otras instalaciones de Python en PATH :

• 2.7 : /usr/local/bin/python2.7
• 2.7 : /Users/jerzyk/.pyenv/shims/python2.7
• 2.7 : /usr/local/bin/python2.7
• 2.7 : /usr/local/bin/python2.7
• 2.7 : /usr/bin/python2.7
• 3.6 : /Users/jerzyk/.pyenv/shims/python3.6
• 3.7 : /Users/jerzyk/.pyenv/versions/3.7.0/bin/python3.7m
• 3.7 : /Users/jerzyk/.pyenv/versions/3.7.0/bin/python3.7
• 3.7 : /Users/jerzyk/.pyenv/shims/python3.7
• 3.7 : /usr/local/bin/python3.7

• 3.7 : /usr/local/bin/python3.7

• 3.7.0 : /Users/jerzyk/.pyenv/versions/3.7.0/bin/python

• 3.7.0 : /Users/jerzyk/.pyenv/shims/python
• 2.7.15 : /usr/local/bin/python
• 2.7.15 : /usr/local/bin/python
• 2.7.10 : /usr/bin/python
• None : /Users/jerzyk/.pyenv/shims/python2
• 2.7.15 : /usr/local/bin/python2
• 2.7.15 : /usr/local/bin/python2
• 3.7.0 : /Users/jerzyk/.pyenv/versions/3.7.0/bin/python3
• 3.7.0 : /Users/jerzyk/.pyenv/shims/python3
• 3.7.0 : /usr/local/bin/python3
• 3.7.0 : /usr/local/bin/python3

PEP 508 Información:

{'implementation_name': 'cpython',
 'implementation_version': '3.7.0',
 'os_name': 'posix',
 'platform_machine': 'x86_64',
 'platform_python_implementation': 'CPython',
 'platform_release': '17.7.0',
 'platform_system': 'Darwin',
 'platform_version': 'Darwin Kernel Version 17.7.0: Thu Jun 21 22:53:14 PDT '
                     '2018; root:xnu-4570.71.2~1/RELEASE_X86_64',
 'python_full_version': '3.7.0',
 'python_version': '3.7',
 'sys_platform': 'darwin'}

Variables de entorno del sistema:

• TERM_PROGRAM
• PYENV_ROOT
• TERM
• SHELL
• TMPDIR
• Apple_PubSub_Socket_Render
• TERM_PROGRAM_VERSION
• TERM_SESSION_ID
• PYENV_VERSION
• USER
• SSH_AUTH_SOCK
• PYENV_DIR
• __CF_USER_TEXT_ENCODING
• PATH
• PWD
• LANG
• PGHOST
• ITERM_PROFILE
• PYENV_HOOK_PATH
• XPC_FLAGS
• XPC_SERVICE_NAME
• SHLVL
• HOME
• COLORFGBG
• ITERM_SESSION_ID
• LOGNAME
• PGDATA
• DISPLAY
• COLORTERM
• PYTHONDONTWRITEBYTECODE
• PIP_PYTHON_PATH

Variables de entorno específicas de Pipenv:

Variables de entorno específicas de depuración:

• PATH : /Users/jerzyk/.pyenv/versions/3.7.0/bin:/usr/local/Cellar/pyenv/1.2.6/libexec:/Users/jerzyk/.pyenv/shims:/usr/local/bin:/usr/local/sbin:/Applications/Xcode.app/Contents/Developer/usr/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/opt/X11/bin:/usr/local/MacGPG2/bin
• SHELL : /bin/bash
• LANG : en_US.UTF-8
• PWD : /Users/jerzyk/tmp

Contenido de Pipfile ('/Users/jerzyk/tmp/Pipfile'):

[[source]]
url = "https://pypi.org/simple"
verify_ssl = true
name = "pypi"

[packages]
flower = "*"

[dev-packages]

[requires]
python_version = "3.7"

Contenido de Pipfile.lock ('/Users/jerzyk/tmp/Pipfile.lock'):

{
    "_meta": {
        "hash": {
            "sha256": "e224f38cf9f41ab210a6546986368cfee3a9087dc1c0d756024e2f6aea1f7e9a"
        },
        "pipfile-spec": 6,
        "requires": {
            "python_version": "3.7"
        },
        "sources": [
            {
                "name": "pypi",
                "url": "https://pypi.org/simple",
                "verify_ssl": true
            }
        ]
    },
    "default": {
        "amqp": {
            "hashes": [
                "sha256:073dd02fdd73041bffc913b767866015147b61f2a9bc104daef172fc1a0066eb",
                "sha256:eed41946890cd43e8dee44a316b85cf6fee5a1a34bb4a562b660a358eb529e1b"
            ],
            "version": "==2.3.2"
        },
        "babel": {
            "hashes": [
                "sha256:6778d85147d5d85345c14a26aada5e478ab04e39b078b0745ee6870c2b5cf669",
                "sha256:8cba50f48c529ca3fa18cf81fa9403be176d374ac4d60738b839122dfaaa3d23"
            ],
            "version": "==2.6.0"
        },
        "billiard": {
            "hashes": [
                "sha256:ed65448da5877b5558f19d2f7f11f8355ea76b3e63e1c0a6059f47cfae5f1c84"
            ],
            "version": "==3.5.0.4"
        },
        "celery": {
            "hashes": [
                "sha256:77dab4677e24dc654d42dfbdfed65fa760455b6bb563a0877ecc35f4cfcfc678",
                "sha256:ad7a7411772b80a4d6c64f2f7f723200e39fb66cf614a7fdfab76d345acc7b13"
            ],
            "version": "==4.2.1"
        },
        "flower": {
            "hashes": [
                "sha256:a7a828c2dbea7e9cff1c86d63626f0eeb047b1b1e9a0ee5daad30771fb51e6d0"
            ],
            "index": "pypi",
            "version": "==0.9.2"
        },
        "kombu": {
            "hashes": [
                "sha256:86adec6c60f63124e2082ea8481bbe4ebe04fde8ebed32c177c7f0cd2c1c9082",
                "sha256:b274db3a4eacc4789aeb24e1de3e460586db7c4fc8610f7adcc7a3a1709a60af"
            ],
            "version": "==4.2.1"
        },
        "pytz": {
            "hashes": [
                "sha256:a061aa0a9e06881eb8b3b2b43f05b9439d6583c206d0a6c340ff72a7b6669053",
                "sha256:ffb9ef1de172603304d9d2819af6f5ece76f2e85ec10692a524dd876e72bf277"
            ],
            "version": "==2018.5"
        },
        "tornado": {
            "hashes": [
                "sha256:1c0816fc32b7d31b98781bd8ebc7a9726d7dce67407dc353a2e66e697e138448",
                "sha256:4f66a2172cb947387193ca4c2c3e19131f1c70fa8be470ddbbd9317fd0801582",
                "sha256:5327ba1a6c694e0149e7d9126426b3704b1d9d520852a3e4aa9fc8fe989e4046",
                "sha256:6a7e8657618268bb007646b9eae7661d0b57f13efc94faa33cd2588eae5912c9",
                "sha256:a9b14804783a1d77c0bd6c66f7a9b1196cbddfbdf8bceb64683c5ae60bd1ec6f",
                "sha256:c58757e37c4a3172949c99099d4d5106e4d7b63aa0617f9bb24bfbff712c7866",
                "sha256:d8984742ce86c0855cccecd5c6f54a9f7532c983947cff06f3a0e2115b47f85c"
            ],
            "markers": "python_version != '3.2.*' and python_version != '3.1.*' and python_version != '3.3.*' and python_version != '3.0.*' and python_version >= '2.7'",
            "version": "==5.1"
        },
        "vine": {
            "hashes": [
                "sha256:52116d59bc45392af9fdd3b75ed98ae48a93e822cee21e5fda249105c59a7a72",
                "sha256:6849544be74ec3638e84d90bc1cf2e1e9224cc10d96cd4383ec3f69e9bce077b"
            ],
            "version": "==1.1.4"
        }
    },
    "develop": {}
}