Update Python used in Windows CI to 3.8

[c-w]

Fix CI on Windows

Description

The Travis CI build on Windows is currently failing (e.g. see 5073.4 or 5074.4) due to an error installing Python via Chocolatey.

This pull request fixes the CI by updating the build to use the latest Python published to Chocolatey (3.8). This also required updating the cryptography dependency to the latest version (2.8) since the previously pinned version (2.6.1) doesn't include a wheel for Python 3.8 on Windows.

Status

• done, ready for review

Checklist

• Code linting (n/a, infrastructure-only change)
• Documentation (n/a, infrastructure-only change)
• Tests (CI passed)
• ICLA (committer)

[codecov-io]

Codecov Report

Merging #1404 into trunk will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            trunk    #1404   +/-   ##
=======================================
  Coverage   86.45%   86.45%           
=======================================
  Files         366      366           
  Lines       76751    76751           
  Branches     7527     7527           
=======================================
  Hits        66352    66352           
  Misses       7532     7532           
  Partials     2867     2867

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[Kami]

Thanks for the fix.

If I understand correctly, chocolatey on Travis only supports a single (latest) version of Python 3?

[c-w]

Turns out this was a temporary (but persistent over multiple hours) bug with Chocolatey yesterday which failed installs when --version was specified. While the issue now seems resolved (e.g. see successful build which pins Python 3.8.1 and one which pins Python 3.7.5), I'd say that it's still worth merging this PR to get the Windows CI updated to the latest Python version. I'll rename the PR before merge.