build(deps): update coverage requirement from ~=5.5 to ~=6.0

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-5.5...6.0)

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

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

requirements-test.txt

@@ -1,5 +1,5 @@
 pytest
-coverage~=5.5
+coverage~=6.0
 codecov
 pytest-cov
 pytest-benchmark~=3.4.1