Switched from Travis CI to Github Actions

[agronholm]

This replaces the Travis CI configuration with Github Actions workflows which test the master branch and all pull requests.
In addition, they also automatically build and upload binary wheels and a source distribution to PyPI when you push a tag matching a specific pattern.

[coveralls]

Coverage decreased (-8.9%) to 90.856% when pulling 9e26cdb on workflow into d92b4ba on master.

[Sekenre]

@agronholm Could you change the thresholds on coveralls to allow -1% or -2% change in coverage to pass? It generates lots of false negatives particularly when deleting code.

[agronholm]

Sure. In this particular case though, it's detecting 0% coverage which is nuts, and I don't know why.

[agronholm]

I've switched the coverage decrease threshold to 3%.

[agronholm]

After running the test suite: Coverage.py warning: No data was collected. (no-data-collected). This is despite a detailed coverage chart being printed which indicates 90% coverage. WTF?

[agronholm]

Ok, that takes care of the coverage problem. The reason coverage fell that much was because I did not include Python 2.7 in the test array. I'll add that too I suppose.

[agronholm]

I fixed the 2.7 test errors but the highest coveralls version supported on 2.7 is so old it won't work with Github Actions. Bummer. How do you want to handle this?

[Sekenre]

I think tag the current master and update documentation to say this will be the last version tested on 2.7, then merge this PR. How does that sound?

[agronholm]

Sounds good. You may want to drop support for all Pythons older than 3.6 at the same time since 3.5 has passed its EOL date.

[agronholm]

Note that you should really drop 2.x compatibility code at the same time; not doing that will lead to 2.x breakage. I can make a PR for that if you want.

[Sekenre]

If you have time that would be awesome!

[agronholm]

I can probably do that for the Python code but not for the C code as I didn't write any of that.

[waveform80]

Apologies I've been absent for a while! I can try and take a look at integrating coverage for the C bits if wanted; it looks like coveralls has (community contributed) support for gcov output which is what my little coverage script used (lcov is just a gcov front-end).

I think there's still some 3.5 compatibility bits in the C code too which I can excise at the same time.

[agronholm]

I guess it's ready to be merged now. I will follow up with a PR that removes other py2.7 specifics.