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](coveragepy/coveragepy@coverage-5.4...coverage-5.5)

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

Author: dependabot[bot]

requirements_test.txt

@@ -1,5 +1,5 @@
 aresponses==2.1.4
-coverage==5.4
+coverage==5.5
 flake8==3.8.4
 flake8-docstrings==1.5.0
 isort==5.8.0