Drop Travis CI and migrate Coveralls

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.

.coveragerc

@@ -3,3 +3,6 @@ omit =
   src/blib2to3/*
   tests/data/*
   */site-packages/*
+
+[run]
+relative_files = True

.github/workflows/test.yml

@@ -34,3 +34,34 @@ jobs:
       - name: Unit tests
         run: |
           tox -e py
+
+      - name: Publish coverage to Coveralls
+        # If pushed / is a pull request against main repo AND
+        # we're running on Linux (this action only supports Linux)
+        if:
+          ((github.event_name == 'push' && github.repository == 'psf/black') ||
+          github.event.pull_request.base.repo.full_name == 'psf/black') && matrix.os ==
+          'ubuntu-latest'
+
+        uses: AndreMiras/coveralls-python-action@v20201129
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          parallel: true
+          flag-name: py${{ matrix.python-version }}-${{ matrix.os }}
+          debug: true
+
+  coveralls-finish:
+    needs: build
+    # If pushed / is a pull request against main repo
+    if:
+      (github.event_name == 'push' && github.repository == 'psf/black') ||
+      github.event.pull_request.base.repo.full_name == 'psf/black'
+
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Coveralls finished
+        uses: AndreMiras/coveralls-python-action@v20201129
+        with:
+          parallel-finished: true
+          debug: true

README.md

@@ -3,7 +3,6 @@
 <h2 align="center">The Uncompromising Code Formatter</h2>
 
 <p align="center">
-<a href="https://travis-ci.com/psf/black"><img alt="Build Status" src="https://travis-ci.com/psf/black.svg?branch=master"></a>
 <a href="https://github.com/psf/black/actions"><img alt="Actions Status" src="https://github.com/psf/black/workflows/Test/badge.svg"></a>
 <a href="https://github.com/psf/black/actions"><img alt="Actions Status" src="https://github.com/psf/black/workflows/Primer/badge.svg"></a>
 <a href="https://black.readthedocs.io/en/stable/?badge=stable"><img alt="Documentation Status" src="https://readthedocs.org/projects/black/badge/?version=stable"></a>

docs/conf.py

@@ -314,7 +314,6 @@ def process_sections(
     "show_powered_by": True,
     "fixed_sidebar": True,
     "logo": "logo2.png",
-    "travis_button": True,
 }