Allow using tox to reformat the working copy

By default, tox should check that the code is correctly formatted
CI-style. However it can be useful to use it to *fix* the code as
well. By using `--diff` as an overridable default,

    tox -eblack --

becomes an easy way to reformat the working copy. Technically it's not
that much shorter than `ruff format`, but if the previous command was
`tox -eblack` (to check formatting) then it becomes a smaller change,
and I don't have to remember the ruff invocation.

tox.ini

@@ -36,7 +36,7 @@ commands = ruff check {posargs}
 [testenv:black]
 package = skip
 deps = ruff
-commands = ruff format --diff {posargs:.}
+commands = ruff format {posargs:--diff}
 
 [testenv:typecheck]
 package = skip