Fixed a regression introduced in version 1.2 where a refactor
of the :class:`.SQLAlchemyError` base exception class introduced an
inappropriate coercion of a plain string message into Unicode under
python 2k, which is not handled by the Python interpreter for characters
outside of the platform's encoding (typically ascii).  The
:class:`.SQLAlchemyError` class now passes a bytestring through under
Py2K for ``__str__()`` as is the behavior of exception objects in general
under Py2K, does a safe coercion to unicode utf-8 with
backslash fallback for ``__unicode__()``.  For Py3K the message is
typically unicode already, but if not is again safe-coerced with utf-8
with backslash fallback for the ``__str__()`` method.

Fixes: #4429
Change-Id: I2289da3f2c45c7d0041fa43d838958f7614defc3
(cherry picked from commit 2db54ee92ebd0970f52b271e152a6df9b563693f)

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 bug where a :class:`.Sequence` would be dropped explicitly before any
:class:`.Table` that refers to it, which breaks in the case when the
sequence is also involved in a server-side default for that table, when
using :meth:`.MetaData.drop_all`.   The step which processes sequences
to be dropped via non server-side column default functions is now invoked
after the table itself is dropped.

Change-Id: I185f2cc76d2011ad4dd3ba9bde5d8aef0ec335ae
Fixes: #4300
(cherry picked from commit 532566ba1f28ff8a6afa6eacc10c59eb918501f6)

Fixed a reference leak issue where the values of the parameter dictionary
used in a statement execution would remain referenced by the "compiled
cache", as a result of storing the key view used by Python 3 dictionary
keys().  Pull request courtesy Olivier Grisel.

Change-Id: Icfb0f38111a165780f6dd3e4e3382a03df79ce26
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/441
(cherry picked from commit 30b02003a70f37aa83e20de6229afe2a3600b648)

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

Fixed bug in cx_Oracle disconnect detection, used by pre_ping and other
features, where an error could be raised as DatabaseError which includes a
numeric error code; previously we weren't checking in this case for a
disconnect code.

Change-Id: I359bb5ede25a4726ea632b81af83c6391f405ae1
Fixes: #4182

Change-Id: Ida0d01ae9bcc0573b86e24fddea620a38c962822