Remove Travis CI

[hugovk]

Travis CI have changed their pricing model and 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.

[codecov-io]

Codecov Report

Merging #179 (93119f2) into master (184269e) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #179   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          569       569           
=========================================
  Hits           569       569           

Flag	Coverage Δ
GHA_Ubuntu	100.00% <ø> (ø)
GHA_Windows	100.00% <ø> (ø)
GHA_macOS	100.00% <ø> (ø)

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