Don't try to download windows exe/msi and mac universal pkg if it doesn't exists for python version requested

[jmarrec]

Only some minor versions of python have an actual windows .exe / .msi installer.

eg:

• this does https://www.python.org/downloads/release/python-3810/
• this doesn't https://www.python.org/downloads/release/python-3811/

[mayeut]

Rather than silently ignore some network failures that would lead to the version being skipped, the json returned by https://www.python.org/api/v2/downloads/release/?is_published=true/https://www.python.org/api/v2/downloads/release_file/?release=... should be parsed to check if the package exists or not. This shall probably be done while generating the build matrix.