Bump coverage from 4.5.4 to 5.2.1 (#290)

Bumps [coverage](https://github.com/nedbat/coveragepy) from 4.5.4 to 5.2.1.
- [Release notes](https://github.com/nedbat/coveragepy/releases)
- [Changelog](https://github.com/nedbat/coveragepy/blob/master/CHANGES.rst)
- [Commits](nedbat/coveragepy@coverage-4.5.4...coverage-5.2.1)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

tests/requirements.txt

@@ -1,4 +1,4 @@
-coverage==4.5.4
+coverage==5.2.1
 ddt
 mock
 pytest>=4.6.0