Remove Travis CI and add PyPy3 and coverage to GHA

[hugovk]

Travis CI have changed their pricing model and have introduced limits for open source:

• https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing
• https://www.jeffgeerling.com/blog/2020/travis-cis-new-pricing-plan-threw-wrench-my-open-source-works

Many projects are migrating to GitHub Actions:

• Migrate off of travis-ci python-trio/trio#1785
• Migrate off of Travis CI urllib3/urllib3#2033
• The new pricing model for Travis CI python-pillow/Pillow#5028
• etc.

We already have tests on GHA and Travis, all that was missing was Codecov upload (it was initially not supported for GHA but is now) and PyPy3.

This PR adds those to GHA (although PyPy3 is still failing, same as #102 (comment)).

[codecov]

Codecov Report

Merging #104 (2aaefad) into master (b008066) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #104   +/-   ##
=======================================
  Coverage   94.05%   94.05%           
=======================================
  Files           6        6           
  Lines         353      353           
  Branches       36       36           
=======================================
  Hits          332      332           
  Misses         12       12           
  Partials        9        9           

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[ofek]

Thank you!!!