Update coverage requirement to <5

Updates the requirements on [coverage](https://bitbucket.org/ned/coveragepy) to permit the latest version.
- [Changelog](https://bitbucket.org/ned/coveragepy/src/master/CHANGES.rst)
- [Commits](https://bitbucket.org/ned/coveragepy/commits/tag/coverage-4.5.1)

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

Author: dependabot-support

requirements-dev.txt

@@ -7,7 +7,7 @@ tox>=2.9.1
 pytest-rerunfailures>=3.1
 
 # code coverage
-coverage<4
+coverage<5
 codecov>=1.6.3
 pytest-cov>=2.2.1