[master] Add manylinux wheels for python 3.8

[tlestang]

Enabling and triggering the build of Python 3.8 wheels for GNU/Linux platforms.
This is effectively applying to master the changes applied to develop with #1197 and #1198

[valentinsulzer]

Looks good once the tests pass ...

[tlestang]

It's just the random Homebrew error, not much I can for now I'm afraid.
(Homebrew/homebrew-core#61772)

Just to provide an update, we've reached out to GitHub again and their engineering team is looking at this once again.
However, like previously mentioned, there's nothing we can do on our end to fix this issue, so please sit tight.
We maintainers are crossing our fingers as hard as everyone else is at this time.