chore: update pytest-cov requirement from ==2.* to >=2,<4

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@v2.0.0...v3.0.0)

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

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

Author: dependabot[bot]

setup.py

@@ -44,7 +44,7 @@ def find_version(*file_paths):
         "docutils==0.*",
         "guzzle-sphinx-theme==0.*",
         "m2r2==0.*",
-        "pytest-cov==2.*",
+        "pytest-cov>=2,<4",
         "sphinx==3.*",
         "sphinxcontrib-log-cabinet==1.*",
         "sphinxcontrib-websupport==1.*",