chore(deps): update pytest-cov requirement from <6,>=3 to >=3,<7 (#1196)

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@v3.0.0...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_without_asyncio.txt

@@ -1,3 +1,3 @@
 # pip install -r requirements/testing_without_asyncio.txt
 pytest>=6.2.5,<8.4        # https://github.com/tornadoweb/tornado/issues/3375
-pytest-cov>=3,<6
+pytest-cov>=3,<7