Bump coverage from 4.5.4 to 5.0.3

Bumps [coverage](https://github.com/nedbat/coveragepy) from 4.5.4 to 5.0.3.
- [Release notes](https://github.com/nedbat/coveragepy/releases)
- [Changelog](https://github.com/nedbat/coveragepy/blob/master/CHANGES.rst)
- [Commits](https://github.com/nedbat/coveragepy/compare/coverage-4.5.4...coverage-5.0.3)

Signed-off-by: dependabot-preview[bot] <[email protected]>

requirements-dev.txt

@@ -1,4 +1,4 @@
-coverage==4.5.4
+coverage==5.0.3
 ddt==1.2.2
 flake8==3.7.9
 mock==3.0.5