Update pytest-cov requirement from ==2.11.* to ==3.0.*

Updates the requirements on [pytest-cov](https://github.com/pytest-dev/pytest-cov) to permit the latest version.
- [Release notes](https://github.com/pytest-dev/pytest-cov/releases)
- [Changelog](https://github.com/pytest-dev/pytest-cov/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-cov@v2.11.0...v3.0.0)

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

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

Author: dependabot[bot]

requirements-dev.txt

@@ -5,7 +5,7 @@ mongomock==3.22.*
 moto==3.1.16
 mypy==0.790
 pytest==6.2.*
-pytest-cov==2.11.*
+pytest-cov==3.0.*
 pytest-mock==3.5.*
 pytest-vcr==1.0.*
 coveralls==3.0.*