Implemented the SQLAlchemy 2 :func:`.future.create_engine` function which
is used for forwards compatibility with SQLAlchemy 2.   This engine
features always-transactional behavior with autobegin.

Allow execution options per statement execution.  This includes
that the before_execute() and after_execute() events now accept
an additional dictionary with these options, empty if not
passed; a legacy event decorator is added for backwards compatibility
which now also emits a deprecation warning.

Add some basic tests for execution, transactions, and
the new result object.   Build out on a new testing fixture
that swaps in the future engine completely to start with.

Change-Id: I70e7338bb3f0ce22d2f702537d94bb249bd9fb0a
Fixes: #4644

Change-Id: I08440dc25e40ea1ccea1778f6ee9e28a00808235

Fixed an issue whereby if the dialect "initialize" process which occurs on
first connect would encounter an unexpected exception, the initialize
process would fail to complete and then no longer attempt on subsequent
connection attempts, leaving the dialect in an un-initialized, or partially
initialized state, within the scope of parameters that need to be
established based on inspection of a live connection.   The "invoke once"
logic in the event system has been reworked to accommodate for this
occurrence using new, private API features that establish an "exec once"
hook that will continue to allow the initializer to fire off on subsequent
connections, until it completes without raising an exception. This does not
impact the behavior of the existing ``once=True`` flag within the event
system.

Fixes: #4807
Change-Id: Iec32999b61b6af4b38b6719e0c2651454619078c

Replace inspect_getargspec with inspect_getfullargspec
including a compatibility fallback for Py2k and use
getfullargspec fully.

Change-Id: I92bce0aafc37ce1a360b4f61b75f5892d0911c7e

Change-Id: I6a71f4924d046cf306961c58dffccf21e9c03911

Applied on top of a pure run of black -l 79 in
I7eda77fed3d8e73df84b3651fd6cfcfe858d4dc9,  this set of changes
resolves all remaining flake8 conditions for those codes
we have enabled in setup.cfg.

Included are resolutions for all remaining flake8 issues
including shadowed builtins, long lines, import order, unused
imports, duplicate imports, and docstring issues.

Change-Id: I4f72d3ba1380dd601610ff80b8fb06a2aff8b0fe

This is a straight reformat run using black as is, with no edits
applied at all.

The black run will format code consistently, however in
some cases that are prevalent in SQLAlchemy code it produces
too-long lines.   The too-long lines will be resolved in the
following commit that will resolve all remaining flake8 issues
including shadowed builtins, long lines, import order, unused
imports, duplicate imports, and docstring issues.

Change-Id: I7eda77fed3d8e73df84b3651fd6cfcfe858d4dc9

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