Bug 1759042: Add vendored six to `hooks_clang_format.py` r=ahal

The clang-format hook appears to be called directly without Mach, so it
has to manually handle its dependencies.
Since it transitively depends on `six` (through
`mozversioncontrol` > `mozfile`), it should add the vendored `six` to
the `sys.path`.

Differential Revision: https://phabricator.services.mozilla.com/D140769

tools/lint/hooks_clang_format.py

@@ -17,6 +17,7 @@ EXTRA_PATHS = (
     "python/mozversioncontrol",
     "testing/mozbase/mozfile",
     "third_party/python/jsmin",
+    "third_party/python/six",
 )
 sys.path[:0] = [os.path.join(topsrcdir, p) for p in EXTRA_PATHS]