A full rewrite of all imports and pep8 formatting using zimports, black,
commits are broken into sections.

Directories included in this commit:

    lib/sqlalchemy/*.py  lib/sqlalchemy/engine/  lib/sqlalchemy/pool/  lib/sqlalchemy/event/

Change-Id: Idf6a7ba01bacd00fd9271fefd7e1bbab6f91f22b
(cherry picked from commit e85ed21d2cdfcb215ff363592daecf9fe0d9e576)

includes adjustment to setup.py to recognize __version__ correctly

Fixed regression caused in 1.2.3 due to fix from :ticket:`4181` where
the changes to the event system involving :class:`.Engine` and
:class:`.OptionEngine` did not accommodate for event removals, which
would raise an ``AttributeError`` when invoked at the class
level.

Change-Id: I1c9083829d74dd710716d28b0eaca4fa15e86313
Fixes: #4190

Fixed bug where events associated with an :class:`Engine`
at the class level would be doubled when the
:meth:`.Engine.execution_options` method were used.  To
achieve this, the semi-private class :class:`.OptionEngine`
no longer accepts events directly at the class level
and will raise an error; the class only propagates class-level
events from its parent :class:`.Engine`.   Instance-level
events continue to work as before.

The comments present another way of doing this where we would
copy events from the parent engine at option time rather
than linking the event listeners, but this would be a behavioral
change that adding new events to the parent engine would not
take effect for an already-created OptionEngine.

Change-Id: Id128516f54103fbad9a2210d6571eceb59c8b0cb
Fixes: #4181

Change-Id: I3ef36bfd0cb0ba62b3123c8cf92370a43156cf8f

Change-Id: I4e8c2aa8fe817bb2af8707410fa0201f938781de

didn't implement ``__slots__``, and therefore meant all subclasses
of that class didn't either, negating the rationale for ``__slots__``
to be in use.  Didn't cause any issue except on IronPython
which apparently does not implement ``__slots__`` behavior compatibly
with cPython.
Fixes #3494