Bump coverage from 4.5.4 to 5.1 in /src/requirements (#508)

Bumps [coverage](https://github.com/nedbat/coveragepy) from 4.5.4 to 5.1.
- [Release notes](https://github.com/nedbat/coveragepy/releases)
- [Changelog](https://github.com/nedbat/coveragepy/blob/master/CHANGES.rst)
- [Commits](https://github.com/nedbat/coveragepy/compare/coverage-4.5.4...coverage-5.1)

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

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

src/requirements/dev.txt

@@ -2,7 +2,7 @@
 
 django-debug-toolbar==2.2
 factory_boy==2.12.0
-coverage==4.5.4
+coverage==5.1
 codecov==2.1.4
 black==19.10b0
 pre-commit==2.4.0