tox: use the same wheel for all environments

Since pytest wheel is "universal" (not different between Python versions
or implementations or anything), the same wheel can be reused. See:

https://tox.wiki/en/stable/config.html#wheel_build_env
https://tox.wiki/en/stable/upgrading.html#universal-wheels

Author: bluetech

tox.ini

@@ -92,6 +92,9 @@ deps =
     xdist: pytest-xdist>=2.1.0
     xdist: -e .
     {env:_PYTEST_TOX_EXTRA_DEP:}
+# Can use the same wheel for all environments.
+package = wheel
+wheel_build_env = .pkg
 
 [testenv:linting]
 description =