Update pytest-cov requirement from ==2.12.* 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@v2.12.0...v3.0.0)

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

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

requirements/development.txt

@@ -6,5 +6,5 @@ black
 feedparser>=6.0,<6.1
 flake8>=3.8,<4.1
 pre-commit>=2.10,<2.17
-pytest-cov==2.12.*
+pytest-cov==3.0.*
 validator-collection>=1.5,<1.6