Bump coverage from 5.3.1 to 5.5 (#361)

Bumps [coverage](https://github.com/nedbat/coveragepy) from 5.3.1 to 5.5.
- [Release notes](https://github.com/nedbat/coveragepy/releases)
- [Changelog](https://github.com/nedbat/coveragepy/blob/master/CHANGES.rst)
- [Commits](nedbat/coveragepy@coverage-5.3.1...coverage-5.5)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

test-requirements.txt

@@ -2,4 +2,4 @@ flake8==3.8.4
 black==20.8b1
 pre-commit==2.9.2
 django-nose==1.4.7
-coverage==5.3.1
+coverage==5.5