Use fully vendored getfullargspec

Use fully vendored getfullargspec

Replaced the Python compatbility routines for ``getfullargspec()`` with a
fully vendored version from Python 3.3.  Originally, Python was emitting
deprecation warnings for this function in Python 3.8 alphas.  While this
change was reverted, it was observed that Python 3 implementations for
``getfullargspec()`` are an order of magnitude slower as of the 3.4 series
where it was rewritten against ``Signature``.  While Python plans to
improve upon this situation, SQLAlchemy projects for now are using a simple
replacement to avoid any future issues.

Fixes: #4674
Change-Id: I1ab8729c4072634db80c79bb7bc44197dc5e7114