Update coverage requirement from <5.6,>=4.0 to >=4.0,<6.2

Updates the requirements on [coverage](https://github.com/nedbat/coveragepy) to permit the latest version.
- [Release notes](https://github.com/nedbat/coveragepy/releases)
- [Changelog](https://github.com/nedbat/coveragepy/blob/master/CHANGES.rst)
- [Commits](nedbat/coveragepy@coverage-4.0...6.1.1)

---
updated-dependencies:
- dependency-name: coverage
  dependency-type: direct:production
...

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

test-requirements.txt

@@ -8,6 +8,6 @@ pytest-cov
 flake8
 coveralls
 codeclimate-test-reporter
-coverage>=4.0,<5.6
+coverage>=4.0,<6.2
 prometheus_client
 versioneer