Docs: napoleon is now part of Sphinx

According to the [sphinxcontrib-napoleon docs](https://pypi.org/project/sphinxcontrib-napoleon/), as of Sphinx 1.3, napoleon is now packaged as part of Sphinx as `sphinx.ext.napoleon`.

Therefore, the below line should be updated:
https://github.com/google/pygtrie/blob/64ee0836f41a59919ecf8a59b0c7e2f7f1b8c5ba/conf.py#L27