Bump coverage from 4.2 to 5.0.4

Bumps [coverage](https://github.com/nedbat/coveragepy) from 4.2 to 5.0.4.
- [Release notes](https://github.com/nedbat/coveragepy/releases)
- [Changelog](https://github.com/nedbat/coveragepy/blob/master/CHANGES.rst)
- [Commits](nedbat/coveragepy@coverage-4.2...coverage-5.0.4)

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

requirements/dev.txt

@@ -1,4 +1,4 @@
-coverage==4.2
+coverage==5.0.4
 mock==3.0.5 # last version compatible with python2.7
 pytest==4.6.9 # last version compatible with python2.7
 pytest-mock == 2.0.0