Update pytest-cov requirement from <3.0.0,>=2.6.1 to >=2.6.1,<4.0.0

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@v2.6.1...v3.0.0)

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

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

setup.py

@@ -36,7 +36,7 @@
     'openfisca-country-template >= 3.10.0, < 4.0.0',
     'openfisca-extension-template >= 1.2.0rc0, < 2.0.0',
     'pylint == 2.10.2',
-    'pytest-cov >= 2.6.1, < 3.0.0',
+    'pytest-cov >= 2.6.1, < 4.0.0',
     ] + api_requirements
 
 setup(