Spyder: Creating a new project makes the path with mixed slashes

Issue Report Checklist

• [x] Searched the issues page for similar reports
• [x] Completed the Problem Description, Steps to Reproduce and Version sections below

Problem Description

Happens when you select a different Location

What steps reproduce the problem?

1. Select a different Location from the project creation dialog
2. Type a name for the project

What is the expected output? What do you see instead?

A proper path and not mixed slashes

Versions

• Spyder version: 3.2.8
• Python version: 3.6.4
• Qt version: 5.6.2
• PyQt version: 5
• Operating System name/version: Windows

Dependencies

IPython >=4.0 : 6.2.1 (OK)
cython >=0.21 : None (NOK)
jedi >=0.9.0 : 0.11.1 (OK)
nbconvert >=4.0 : 5.3.1 (OK)
numpy >=1.7 : 1.14.1 (OK)
pandas >=0.13.1 : 0.23.0 (OK)
pycodestyle >=2.3: 2.3.1 (OK)
pyflakes >=0.6.0 : 1.6.0 (OK)
pygments >=2.0 : 2.2.0 (OK)
pylint >=0.25 : 1.8.2 (OK)
qtconsole >=4.2.0: 4.3.1 (OK)
rope >=0.9.4 : 0.10.7 (OK)
sphinx >=0.6.6 : 1.7.5 (OK)
sympy >=0.7.3 : None (NOK)