Update coverage requirement from ==5.2.* to ==5.5

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](nedbat/coveragepy@coverage-5.2...coverage-5.5)

---
updated-dependencies:
- dependency-name: coverage
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements-test.txt

@@ -6,7 +6,7 @@ codespell==1.17.1
 
 jsonschema==3.2.0
 pylint==1.9.1
-coverage==5.2.*
+coverage==5.5
 doc8==0.8.1
 
 # optional dependencies