build(deps-dev): update pytest-cov requirement

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@v2.3.1...v6.0.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

setup.py

@@ -19,7 +19,7 @@
             'aioresponses>=0.6.2',
             'coverage>=4.2',
             'pytest>=3.0.3',
-            'pytest-cov>=2.3.1,<2.6',
+            'pytest-cov>=2.3.1,<6.1',
             'parametrize>=0.1.1'
         ],
         'dev': [