deps(dev): 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:development
...

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

setup.py

@@ -44,7 +44,7 @@
     ],
     extras_require={
         "tests": [
-            "pytest-cov>=3,<6",
+            "pytest-cov>=3,<7",
             "pytest-instafail==0.5.*",
             "pytest-mock==3.*",
             "pytest-watch==4.*",