Update coverage requirement from ~=6.3.3 to ~=6.4 (#1573)

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@6.3.3...6.4)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements_test.txt

@@ -2,7 +2,7 @@
 -r requirements_test_pre_commit.txt
 contributors-txt>=0.7.4
 coveralls~=3.3
-coverage~=6.3.3
+coverage~=6.4
 pre-commit~=2.19
 pytest-cov~=3.0
 contributors-txt>=0.7.3