Pip-tools: Add python 3.8 support

What's the problem this feature will solve?

Python 3.8 is released, so it's time to support it.

Describe the solution you'd like

1. add "py37" env to tox.ini
2. remove 3.8-dev from .travis.yml
3. add "Programming Language :: Python :: 3.8" classifier to setup.py
4. add "3.8" dimension to .travis.yml (supported, see https://travis-ci.community/t/add-python-3-8-support/5463)
5. add "py37" dimension to .appveyor.yml (not supported yet, but will be on the nex image update, tracking issue: https://github.com/appveyor/ci/issues/3142)
6. add "3.8" to python-version list in .github/workflows/cron.yml (not supported yet, tracking issue: https://github.com/actions/setup-python/issues/30)

Alternative Solutions

N/A

Additional context

https://discuss.python.org/t/python-3-8-0-is-now-available/2478