Remove support for Python 2 and require > 3.6

[Bomme]

I used pyupgrade to do an automatic upgrade. Then looked through the modules to remove code that explicitly mentioned Python 2.

I don't have any experience with travis, so I guessed how the new configuration could look like :)

Closes #112.

[Bomme]

I found two things that I did not touch yet which are probably not required anymore in py3:

            # In Python 3, result of reading from stderr is bytes.
            if isinstance(line, bytes):
                line = line.decode('utf8', 'ignore')

and

# Python 3.4 added support for 24-bit (3-byte) samples.
SUPPORTED_WIDTHS = (1, 2, 3, 4)

But I didn't want to change too many things at once.

[Bomme]

I replaced travis with GitHub Actions and tests are passing. (I guess the Actions appear in this repo after this is merged. For now you can check them in my fork)

I guess a new major version would be the correct way. This gives the possibility of including other breaking changes and only do one release (if you plan to do any other breaking changes)

[sampsyo]

Nice!! Yes, it does seem just fine to leave some of that stuff to future PRs; picking the low-hanging fruit seems like the way to go.

And thank you for setting up GitHub Actions!!! This is super great! I'll merge this now and bump the version.