Add GitHub actions for testing

[joaomcteixeira]

Closes #100

• Tests Ubuntu, MacOS, Windows
• Tests python 2.7, 3.6 to 3.9
• Runs UnitTests on all platforms/versions (sends report to CodeCov)
• tests package build and bump2version on all platforms/versions
• Updates README.rst badges

[joaomcteixeira]

Build is passing well, but tests are failing while passing locally. Git actions is closing I/O before time

ValueError: I/O operation on closed file

needs research...

[codecov[bot]]

Codecov Report

Merging #108 (2b3f199) into master (4cbefcc) will increase coverage by 0.83%. The diff coverage is 97.42%.

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
+ Coverage   82.34%   83.18%   +0.83%     
==========================================
  Files          47       48       +1     
  Lines        3711     3878     +167     
  Branches      767      798      +31     
==========================================
+ Hits         3056     3226     +170     
+ Misses        465      463       -2     
+ Partials      190      189       -1     

Impacted Files	Coverage Δ
pdbtools/pdb_fetch.py	0.00% <0.00%> (ø)
pdbtools/pdb_tidy.py	82.01% <91.30%> (+7.21%)	:arrow_up:
pdbtools/pdb_selaltloc.py	92.05% <96.71%> (+1.90%)	:arrow_up:
pdbtools/__init__.py	100.00% <100.00%> (ø)
pdbtools/pdb_b.py	85.18% <100.00%> (+0.18%)	:arrow_up:
pdbtools/pdb_chain.py	84.61% <100.00%> (+0.19%)	:arrow_up:
pdbtools/pdb_chainbows.py	87.50% <100.00%> (+0.19%)	:arrow_up:
pdbtools/pdb_chainxseg.py	94.54% <100.00%> (+0.10%)	:arrow_up:
pdbtools/pdb_chkensemble.py	87.64% <100.00%> (+0.14%)	:arrow_up:
pdbtools/pdb_delchain.py	84.81% <100.00%> (+0.19%)	:arrow_up:
... and 38 more

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[JoaoRodrigues]

OK, summary after a ton of tinkering:

1. Github Actions are utterly broken when it comes to handling TTYs. See https://github.com/actions/runner/issues/241
2. Because of the above, I had to disable Win testing.
3. Separated coverage/linting/building/testing to avoid redundancy and help manage different OSes.
4. Some tests are failing on Python 2.7, @joaomcteixeira: they are about pdb_selaltloc!
5. Added flake8 linting to CI.