Update pytest-cov requirement from <6,>=2 to >=2,<7 (#1584)

Updates the requirements on [pytest-cov](https://github.com/pytest-dev/pytest-cov) to permit the latest version.
- [Changelog](https://github.com/pytest-dev/pytest-cov/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-cov@v2.0.0...v6.0.0)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Michael Brooks <mbrooks@slack-corp.com>

requirements/testing.txt

@@ -2,7 +2,7 @@
 aiohttp<4  # used for a WebSocket server mock
 pytest>=7.0.1,<9
 pytest-asyncio<1  # for async
-pytest-cov>=2,<6
+pytest-cov>=2,<7
 # while flake8 5.x have issues with Python 3.12, flake8 6.x requires Python >= 3.8.1,
 # so 5.x should be kept in order to stay compatible with Python 3.6/3.7
 flake8>=5.0.4,<8