Remove pin to ancient version of tox

This pin is now standing in the way of having CI install a recent
version of `pytest`, and erecnt version of `pytest-cov` are broken with
the old version of `pytest`.

Co-authored-by: Patrick M. Niedzielski <patrick@pniedzielski.net>
Signed-off-by: Patrick M. Niedzielski <patrick@pniedzielski.net>

Author: godlygeek

requirements-dev.txt

@@ -1,4 +1,5 @@
 -r requirements-lint-docs.txt
 -r requirements-test.txt
 -r requirements-test-coverage.txt
-tox==3.7.0
+tox
+setuptools