[MRG] add cibuildwheel to GitHub actions

[ctb]

Since TravisCI is no longer working well for building wheels, let's give GitHub Actions a try!

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #1244 (61d0e07) into latest (95bd546) will decrease coverage by 54.26%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           latest    #1244       +/-   ##
===========================================
- Coverage   83.31%   29.05%   -54.27%     
===========================================
  Files         103       76       -27     
  Lines        9601     5910     -3691     
===========================================
- Hits         7999     1717     -6282     
- Misses       1602     4193     +2591     

Flag	Coverage Δ
rusttests	?

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

Impacted Files	Coverage Δ
sourmash/cli/plot.py	11.76% <0.00%> (-88.24%)	⬇️
sourmash/lca/lca_db.py	10.31% <0.00%> (-86.62%)	⬇️
sourmash/sig/__main__.py	6.71% <0.00%> (-86.58%)	⬇️
sourmash/cli/lca/index.py	13.63% <0.00%> (-86.37%)	⬇️
sourmash/lca/command_index.py	8.02% <0.00%> (-86.10%)	⬇️
sourmash/cli/gather.py	14.28% <0.00%> (-85.72%)	⬇️
sourmash/cli/lca/classify.py	14.28% <0.00%> (-85.72%)	⬇️
sourmash/cli/lca/summarize.py	14.28% <0.00%> (-85.72%)	⬇️
sourmash/cli/lca/compare_csv.py	14.28% <0.00%> (-85.72%)	⬇️
sourmash/cli/search.py	15.00% <0.00%> (-85.00%)	⬇️
... and 89 more

---

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[luizirber]

I'm taking over this PR and also bringing the remaining checks from Travis (no point in blocking PR merging because Travis is not running our builds)

[ctb]

oh thank god

[luizirber]

I disabled most tests from Travis, but didn't replace them yet with the GH actions version. I'll open a new PR for that, and leave this one only for fixing wheels

(travis tests are not being executed anyway, since we are over our quota =/)

[luizirber]

And codecov is failing because the Python tests are not being reported 😬

Ready for review and merge @ctb

[ctb]

I approve of it, but since I'm the original author I can't officially approve it for merge :). So... let's just merge when tests pass.