build: update coverage requirement from <5 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-3.0b1...coverage-5.5)

requirements-test.txt

@@ -1,5 +1,5 @@
 pytest
-coverage<5
+coverage~=5.5
 codecov
 pytest-mypy
 pytest-cov