Update pytest-cov requirement from ^3.0.0 to ^4.0.0

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](https://github.com/pytest-dev/pytest-cov/compare/v3.0.0...v4.0.0)

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

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

pyproject.toml

@@ -27,7 +27,7 @@ sphinxcontrib-spelling = "^7.3.0"
 toml = "^0.10.2"
 sphinx-rtd-theme = "^1.0.0"
 sphinx-automodapi = "^0.14"
-pytest-cov = "^3.0.0"
+pytest-cov = "^4.0.0"
 pytest-asyncio = "^0.19.0"
 openapi-parser = "^0.2.6"
 autopep8 = "^1.6.0"