Drop Travis CI, add GitHub Actions

[davidbrochart]

No description provided.

[davidbrochart]

In my branch I can see two failures on macos with python 3.8 and 3.9:

../../../hostedtoolcache/Python/3.8.6/x64/lib/python3.8/multiprocessing/popen_spawn_posix.py:47: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

obj = <Process name='Process-4' parent=4583 initial>
file = <_io.BytesIO object at 0x11276ea40>, protocol = None

    def dump(obj, file, protocol=None):
        '''Replacement for pickle.dump() using ForkingPickler.'''
>       ForkingPickler(file, protocol).dump(obj)
E       AttributeError: Can't pickle local object 'ArgumentParser.__init__.<locals>.identity'

../../../hostedtoolcache/Python/3.8.6/x64/lib/python3.8/multiprocessing/reduction.py:60: AttributeError

I will just exclude these ones for now.

[davidbrochart]

Also, the windows tests never start, so I removed windows testing.

[davidbrochart]

Works fine now. I guess this is fine, since we're already testing more environments than previously with Travis CI.
BTW, I dropped python 3.5 and added 3.9.

[blink1073]

Thanks!