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

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] <[email protected]>

requirements/testing.txt

@@ -1,6 +1,6 @@
 # pip install -r requirements/testing.txt
 pytest>=6.2.5,<9
-pytest-cov>=3,<6
+pytest-cov>=3,<7
 Flask>=2.0.3,<4 
 gevent>=22.10.2,<25
 gevent-websocket>=0.10.1,<1