tox: Use pre-commit for pep8 environment

Avoid duplication.

Signed-off-by: Stephen Finucane <stephen@that.guru>
(cherry picked from commit dd8320a)

Author: stephenfin

tox.ini

@@ -37,11 +37,9 @@ commands =
 
 [testenv:pep8]
 deps =
-    black
-    flake8
+    pre-commit
 commands =
-    black --check --quiet {posargs:patchwork manage.py}
-    flake8 {posargs:patchwork manage.py}
+    pre-commit run --all-files --show-diff-on-failure
 
 [flake8]
 # Some rules are ignored as their use makes the code more difficult to read: