Update pytest-cov requirement from ~=2.12 to >=2.12,<4.0 (#1985)

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] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: C. Titus Brown <titus@idyll.org>

setup.cfg

@@ -57,7 +57,7 @@ where = src
 [options.extras_require]
 test =
     pytest>=6.2.4,<7.2.0
-    pytest-cov~=2.12
+    pytest-cov>=2.12,<4.0
     recommonmark
     hypothesis
 demo =