Bump coverage from 4.5.3 to 4.5.4 (#807)

Bump [coverage](https://github.com/nedbat/coveragepy) from 4.5.3 to 4.5.4.
- [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-4.5.3...coverage-4.5.4)

test-requirements.txt

@@ -1,6 +1,6 @@
 pytest==5.0.1
 pytest-cov==2.7.1
-coverage==4.5.3
+coverage==4.5.4
 pre-commit==1.17.0
 responses==0.10.6
 jsonschema==3.0.1