Update CI Python versions where they have updated upstream

[hjmallon]

• Travis make no guarantees for Python version
  stability on their images for macOS.

The version numbers in this file are meant to match those on the CI system. However, the macOS xcode9.4 image on Travis now has 3.7.5.

• I've checked this Git style guide. [Yes]
• I've checked this CMake style guide. [Yes]
• My change will work with CMake 3.2 (minimum requirement for Hunter). [Yes]
• I will try to keep this pull request as small as possible and will try not to mix unrelated features. [Yes]

Here is a Travis run of the failing pkg.pip_numpy branch with this change on top showing macOS build success:

• https://travis-ci.org/hjmallon/hunter/builds/632249789

[bkotzz]

We can lock python versions in our Travis builds like https://docs.travis-ci.com/user/languages/python/#specifying-python-versions. Would that fix this issue?

[hjmallon]

I have just updated this PR with MSYS fix (same thing seems to have happened)

I am now getting all PASSED here

• https://ci.appveyor.com/project/hjmallon/hunter/builds/29877343
• https://ci.appveyor.com/project/hjmallon/hunter/builds/29877343

[hjmallon]

We can lock python versions in our Travis builds like https://docs.travis-ci.com/user/languages/python/#specifying-python-versions. Would that fix this issue?

That Python explicit method is only supported on Linux. MacOS / MSYS Python builds are not supported to the same extent.

Python builds are not available on the macOS and Windows environments.

Quote from the page you linked ^

[bkotzz]

In a previous project I have used this syntax

• os: osx
  osx_image: xcode11.2
  python: 3.7

And this worked fine. I believe we should be able to do the same here

[hjmallon]

In a previous project I have used this syntax

* os: osx
  osx_image: xcode11.2
  python: 3.7

I have just tried with python: 3.6 included in the Travis setup but it still uses 3.7.5. See logs here

https://travis-ci.org/hjmallon/hunter/jobs/632318617

UPDATE: Found a relevant link, but no official info https://travis-ci.community/t/missing-python3-6-on-xcode9-4/6588/7

[bkotzz]

I don’t have a problem with your change, just hoping we could find a way to lock that version so we don’t have to keep updating the code. If we don’t need a particular version, I wonder if we could query the active version rather than hardcoding

[hjmallon]

Yes, it would be nice to avoid the tests starting to fail due to CI updates. Perhaps whoever wrote that travis snippet with the more recent osx_image above might know if/when fixing versions works?

[rbsheth]

It would be nice for us to find and/or lock the right version. For now, I suppose this'll do.