Ci improvements

[gfrances]

This is a work in progress - don't merge.

• Migrate CI pipelines from Travis to GH actions.
• Update Python versions used in tests: run unit tests on all active Python versions (https://www.python.org/downloads/), and style checks on the latest version. In particular,
  • Clean up the mess of requirement versions in the docs extra, and try to update versions to newer releases.
• Deploy package to pypi automatically on merge to master.
• Add mypy code analysis

[codecov-commenter]

Codecov Report

Merging #128 (731be5b) into devel (88cba67) will decrease coverage by 80.13%.
The diff coverage is 9.03%.

@@             Coverage Diff             @@
##            devel     #128       +/-   ##
===========================================
- Coverage   97.71%   17.57%   -80.14%     
===========================================
  Files          51      174      +123     
  Lines        3108    18124    +15016     
  Branches      121     2628     +2507     
===========================================
+ Hits         3037     3186      +149     
- Misses         55    14922    +14867     
  Partials       16       16               

Flag	Coverage Δ
unittests	17.57% <9.03%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)
src/tarski/__init__.py	0.00% <0.00%> (ø)
src/tarski/analysis/csp.py	0.00% <0.00%> (ø)
src/tarski/analysis/csp_schema.py	0.00% <0.00%> (ø)
src/tarski/benchmarks/blocksworld.py	0.00% <0.00%> (ø)
src/tarski/benchmarks/counters.py	0.00% <0.00%> (ø)
src/tarski/dl/concepts.py	0.00% <0.00%> (ø)
src/tarski/dl/factory.py	0.00% <0.00%> (ø)
src/tarski/dl/features.py	0.00% <0.00%> (ø)
src/tarski/errors.py	0.00% <0.00%> (ø)
... and 160 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 134f59e...731be5b. Read the comment docs.