Add support for Python 3.8

[Dunedan]

This should fix failures with pyflakes (#353) and pylint when running prospector with Python 3.8 and marks Python 3.8 as supported by prospector.

Related Issue

#353

How Has This Been Tested?

Aside from running the included tests with Python 3.8 I ran prospector against a large proprietary project which had pylint and pyflakes failing previously when running with Python 3.8.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• My change requires a change to the dependencies
• I have updated the dependencies accordingly
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[coveralls]

Coverage increased (+0.7%) to 74.135% when pulling 8309841 on conhealth:python3.8-support into 00a33d9 on PyCQA:develop.

[chocoelho]

Thanks for the contribution @Dunedan, I should release a new dev version so with this so we others can benefit from it before our next stable release