This fixes mypy from producing this error:

```
$ stubgen -m txredisapi
Traceback (most recent call last):
  File ".../.venv/bin/stubgen", line 8, in <module>
    sys.exit(main())
  File "mypy/stubgen.py", line 1564, in main
  File "mypy/stubgen.py", line 1442, in generate_stubs
  File "mypy/stubgen.py", line 1385, in generate_stub_from_ast
  File "mypy/stubgen.py", line 515, in __init__
TypeError: str object expected; got function
```