Update pytest-cov requirement from ~=3.0 to ~=4.0 (#1817)

Updates the requirements on [pytest-cov](https://github.com/pytest-dev/pytest-cov) to permit the latest version.
- [Release notes](https://github.com/pytest-dev/pytest-cov/releases)
- [Changelog](https://github.com/pytest-dev/pytest-cov/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-cov@v3.0.0...v4.0.0)

---
updated-dependencies:
- dependency-name: pytest-cov
  dependency-type: direct:production
...

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requirements_test.txt

@@ -4,7 +4,7 @@ contributors-txt>=0.7.4
 coveralls~=3.3
 coverage~=6.4
 pre-commit~=2.20
-pytest-cov~=3.0
+pytest-cov~=4.0
 tbump~=6.9.0
 types-typed-ast; implementation_name=="cpython" and python_version<"3.8"
 types-pkg_resources==0.1.3