Bump coverage from 5.3.1 to 5.5

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>

requirements/test.txt

@@ -1,6 +1,6 @@
 
 -r base.txt
-coverage==5.3.1
+coverage==5.5
 cryptography==3.3.1; platform_machine!="i686" and python_version<"3.9" # no 32-bit wheels; no python 3.9 wheels yet
 freezegun==1.0.0
 mypy==0.790; implementation_name=="cpython"