Ship OS-specific wheels for macOS

[webknjaz]

I think at some point I'll complete this task myself unless someone else is going to pick it up.

Motivation: I don't use OS X but I may use pygit2 is some user-facing code and I don't want users to struggle with compiling things correctly.

[jdavid]

Cool. If possible I'd like to use Travis for macOS. Related to issue #902 , eventually we would only use Travis for CI.

[webknjaz]

@jdavid I must warn you that macOS jobs are extremely slow in Travis CI and have even stricter parallelism restrictions which lead to blocking CI flow.

[ngoldbaum]

You can use github actions or azure pipelines to do this as well.

[webknjaz]

Yep, gh actions is a solid choice

[jdavid]

This has been done in PR #988