Bump coverage from 5.4 to 5.5

Bumps [coverage](https://github.com/nedbat/coveragepy) from 5.4 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-5.4...coverage-5.5)

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

requirements/test.txt

@@ -1,6 +1,6 @@
 # Test dependencies go here.
 -r base.txt
 flake8==3.8.4
-coverage==5.4
+coverage==5.5
 django-test-plus==1.4.0
 factory-boy==3.2.0