as the versioned_history example supplies an integer version
counter for the purposes of generating an audit trail, this
counter is also suited to be used for optimistic concurrency
detection using the version_id_col feature.   Build upon
the test that was first added in ac54ba0f2d8df5a76b6852841b6b3321c0e6c0e2
to provide a flag and some basic documentation.

Fixes: #2861
Change-Id: I50236beae4c49b33ada8fdcc4c524273b4e21c75

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

Ensure that the history table sets autoincrement=False, since these values
are copied in all cases; the flag will emit an error as of 1.1 if the
primary key is composite.   Additionally, use the sqlite_autoincrement flag
so that SQLite uses unique primary key identifiers for new rows even if
some rows have been deleted.

Fixes: #3872
Change-Id: I65912eb394b3b69d7f4e3c098f4f948b0a7a5374
Pull-request: https://bitbucket.org/zzzeek/sqlalchemy/pull-requests/93

could encounter empty history, and where a column keyed to an alternate
attribute name would fail to track properly.  Fixes courtesy
Alex Fraser.

mapped columns are re-mapped to
match column names as well as grouping of columns; in particular,
this allows columns that are explicitly grouped in a same-column-named
joined inheritance scenario to be mapped in the same way in the
history mappings, avoiding warnings added in the 0.9 series
regarding this pattern and allowing the same view of attribute
keys.

is currently being supported in addition to nose, and will likely
be preferred to nose going forward.   The nose plugin system used
by SQLAlchemy has been split out so that it works under pytest as
well.  There are no plans to drop support for nose at the moment
and we hope that the test suite itself can continue to remain as
agnostic of testing platform as possible.  See the file
README.unittests.rst for updated information on running tests
with pytest.

The test plugin system has also been enhanced to support running
tests against mutiple database URLs at once, by specifying the ``--db``
and/or ``--dburi`` flags multiple times.  This does not run the entire test
suite for each database, but instead allows test cases that are specific
to certain backends make use of that backend as the test is run.
When using pytest as the test runner, the system will also run
specific test suites multiple times, once for each database, particularly
those tests within the "dialect suite".   The plan is that the enhanced
system will also be used by Alembic, and allow Alembic to run
migration operation tests against multiple backends in one run, including
third-party backends not included within Alembic itself.
Third party dialects and extensions are also encouraged to standardize
on SQLAlchemy's test suite as a basis; see the file README.dialects.rst
for background on building out from SQLAlchemy's test platform.

defaults would prevent history values of NULL from being written.

- pep8 cleanup
- fix tests for "changed" column
- document that "changed" is optional
- changelog