chore(deps): update coverage requirement from ~=5.0 to ~=5.1 (#6920)

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](coveragepy/coveragepy@coverage-5.0...coverage-5.1)

Signed-off-by: dependabot-preview[bot] <[email protected]>

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

Author: dependabot-preview[bot]

requirements/tests.txt

@@ -1,4 +1,4 @@
 -r dev.txt
 
-coverage~=5.0
+coverage~=5.1
 dredd_hooks~=0.2