SQLAlchemy 1.4.27 released on Nov 11, and tox-docs job error become
to occur since then.
`:ref:` attribute introduced from 1.4.27 in docstring of the parent
class of our TypeDecorator cannot be referred.
This patch fixes the error by excluding db/types.py from target of
sphinx-apidoc.
Change-Id: If7944774f2336d8f98124e9934da47b11801c607