Fix redefined-outer-name

src/mailcow/__init__.py:156:8: W0621: Redefining name 'json' from outer scope (line 4) (redefined-outer-name)
src/mailcow/__init__.py:202:34: W0621: Redefining name 'json' from outer scope (line 4) (redefined-outer-name)
src/mailcow/__init__.py:245:8: W0621: Redefining name 'json' from outer scope (line 4) (redefined-outer-name)