Update Python versions in pipeline

[MinchinWeb]

Updates Python 3.7.5 to 3.7.7 and 3.8.0 to 3.8.2.
Also add the alpha 5 build of Python 3.9 as an allowable failure. Poetry doesn't support Python 3.9 yet. See python-poetry/poetry#2154

Checklist

• [ n/a ] The code change is tested and works locally.
• [ ? ] Tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?

[micahellison]

Can this PR update Python versions for all OSes instead of just Windows? Best to keep them all updated at once.

[MinchinWeb]

Do you know where to find a list of the MasOS versions available? Windows versions are coming from Chocolatey, but I'd assumed that the MacOS versions were tied to the base image...

[MinchinWeb]

@micahellison It looks like you can use Homebrew to install custom versions of Python, but that is beyond my experience... see http://www.pyslvs.com/blog/yuan_2018-09-22.html

The Travis documentation doesn't actually say what Python versions are available on what OSX images, just that at least one Python version will always be available.

[micahellison]

On the Mac builds, you can set JRNL_PYTHON_VERSION without having to modify anything else. Python comes pre-installed on macOS, so the build uses pyenv install to install whatever version the JRNL_PYTHON_VERSION is set to.

[wren]

See inline comments.

[MinchinWeb]

@wren @micahellison updated, as requested (other than the MacOS versions).

Changing the MacOS Python versions requires switching to Homebrew for installing Python, which is beyond my expertise.

This should be good to merge!

[wren]

👍