Drop Travis CI and migrate Coveralls (#2186)

Travis CI for Open Source is shutting down in a few weeks so the queue
for jobs is insane due to lower resources. I'm 99.99% sure we don't need
it as our Test, Lint, Docs, Upload / Package, Primer, and Fuzz workflows
are all on GitHub Actions. So even though we *can* migrate to the .com
version with its 1000 free Linux minutes(?), I don't think we need to.

more information here:
- https://blog.travis-ci.com/oss-announcement
- https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing
- https://docs.travis-ci.com/user/migrate/open-source-repository-migration

This commit does the following:
- delete the Travis CI configuration
- add to the GHA test workflows so coverage continues to be recorded
  - tweaked coverage configuration so this wouldn't break
- remove any references to Travis CI in the docs (i.e. readme + sphinx
  docs)

Regarding the Travis CI to GitHub Actions Coveralls transition, the
official action doesn't support the coverage files produced by coverage.py
unfornately. Also no, I don't really know what I am doing so don't @ me
if this breaks :p (well you can, but don't expect me to be THAT useful).

The Coveralls setup has two downfalls AFAIK:
- Only Linux runs are used because AndreMiras/coveralls-python-action
  only supports Linux. Although this isn't a big issue since the Travis
  Coveralls configuration only used Linux data too.
- Pull requests from an internal branch (i.e. one on psf/black) will be
  marked as a push coverage build by Coveralls since our anti-duplicate-
  workflows system runs under the push even for such cases.