Improve testing and building pipeline

[vtemian]

Given the fact that this project supports multiple python versions on multiple platforms, testing for each python version and each platform can be tricky.

The problem arrives because gitfs is highly coupled by pygit2, thus resulting in coupling with python and libgit2. Now, let's say that we pin the version of pygit2 and libgit2 to 0.28.X.

We have to take into account:

• python version (2.7, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8 +)
• operating system (MacOS, Linux)
• distribution (Ubuntu, Debian, Fedora, Centos, Arch) and their versions
• architecture (x86, arm)

Maybe it's a too ambitious plan and for now, is enough to make it work with the stable dependencies, but it might be an interesting issue to tackle.