Bump coverage from 4.5.4 to 5.0

Bumps [coverage](https://github.com/nedbat/coveragepy) from 4.5.4 to 5.0.
- [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-5.0)

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

requirements.txt

@@ -1,4 +1,4 @@
-coverage==4.5.4
+coverage==5.0
 Django==1.11.26 # pyup: <2.0
 django-debug-toolbar==2.1
 docutils==0.15.2