Second run of tox -e py results in a test error for test marked with no_python2 (#2369)

Fixes #2367

tox.ini

@@ -4,6 +4,10 @@ envlist = {,ci-}py{36,37,38,39},fuzz
 [testenv]
 setenv = PYTHONPATH = {toxinidir}/src
 skip_install = True
+# We use `recreate=True` because otherwise, on the second run of `tox -e py`,
+# the `no_python2` tests would run with the Python2 extra dependencies installed.
+# See https://github.com/psf/black/issues/2367.
+recreate = True
 deps =
     -r{toxinidir}/test_requirements.txt
 ; parallelization is disabled on CI because pytest-dev/pytest-xdist#620 occurs too frequently