Remove Python 3.6. Windows w/ Python 3.7 as optional.

[kdewald]

I've encountered plenty of issues of matrix.experimental not being defined. This should solve it.

[kdewald]

@diegoferigo I could not fully reproduce this problem on my machine, so I decided to go with your recommendation to mark the Python 3.7 Windows build as optional to have the CI pass.

I removed setuptools-scm from the examples (as that version is not being used) and left the setuptools version unrestricted, as such older versions should not longer be around and could be helpful to diagnose other problems.

I think the issue with Python 3.7 might be some tool being outdated and causing problems.

[diegoferigo]

Thanks @kdewald for your tests! I suspect the problem is specific to details of the Windows environment in CI, I would not waste more time trying to debug it. If any user will face problems in the future, we can use their system as reference to provide support.