flake8 4.x compat + github actions

[g-as]

I chose to keep flake8 3.x, but it complexifies the test matrix.

[gforcada]

@g-as thanks for the PR! unfortunately, as you can see, travis no longer works for this project 😕 would you be interested in moving the travis CI configuration over to github actions? 🤔 ✨

[g-as]

I'll give it a shot.

Are you ok with dropping 2.7 & 3.5?

[gforcada]

@g-as yes, although I still (shamefully) have some 2.7 code around, but it's time to let the kid fly 😄

[gforcada]

@g-as thanks for all the work! I will create a new release in a few minutes

[g-as]

👍 I'll be around for some adjustments if needed

[gforcada]

And you get not only one but two releases 🎉 4.1.0 and 4.1.1 the second one is to fix the wheel, that it was not py3 only, but py2/3.

[gforcada]

Let me know if you find problems with them!

[g-as]

LOL. Cheers

[g-as]

I made two other small PRs, one to update the README badge (pointing to github actions rather than travis), and one the fix the coveralls coverage upload.