Bump coverage from 5.1 to 5.3

Bumps [coverage](https://github.com/nedbat/coveragepy) from 5.1 to 5.3.
- [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-5.1...coverage-5.3)

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

requirements-dev.txt

@@ -1,4 +1,4 @@
 nose==1.3.7
-coverage==5.1
+coverage==5.3
 rednose==1.3.0
 mock==4.0.2