Fix setup.py to find the namespace package

[pawamoy]

mkdocstrings/mkdocstrings#387

Confirmed it worked by installing the package with pip install . in a venv, moving to a completely different directory, and running from mkdocstrings_handlers import vba in the venv's Python interpreter.

[rudolfbyker]

Very nice, thanks!

We use black to lint our code. It's expecting double-quotes:

     url="https://github.com/AutoActuary/mkdocstrings-vba",
-    packages=setuptools.find_namespace_packages(include=['mkdocstrings_handlers.*']),
+    packages=setuptools.find_namespace_packages(include=["mkdocstrings_handlers.*"]),
     classifiers=[