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-preview[bot] <support@dependabot.com>

requirements-test.txt

@@ -2,7 +2,7 @@
 pytest==6.2.2
 pytest-xdist==2.2.1
 pytest-cov==2.11.1
-coverage==5.4 # pytest-cov
+coverage==5.5 # pytest-cov
 mock==4.0.3
 port-for==0.4
 mirakuru==2.3.0