Repaired support for the :class:`.mssql.DATETIMEOFFSET` datatype on PyODBC,
by adding PyODBC-level result handlers as it does not include native
support for this datatype.  This includes usage of the Python 3 "timezone"
tzinfo subclass in order to set up a timezone, which on Python 2 makes
use of a minimal backport of "timezone" in sqlalchemy.util.

Fixes: #4983
Closes: #4986
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4986
Pull-request-sha: e3a5a8dda01c7580dc93271092c4f9beaac4be11

Change-Id: Ia4c7c0d1971c6c0492515bd5fa0b1799f07fee1c

Fixed issue to workaround SQLite's behavior of assigning "numeric" affinity
to JSON datatypes, first described at :ref:`change_3850`, which returns
scalar numeric JSON values as a number and not as a string that can be JSON
deserialized.  The SQLite-specific JSON deserializer now gracefully
degrades for this case as an exception and bypasses deserialization for
single numeric values, as from a JSON perspective they are already
deserialized.

Also adds a combinatoric fixture for JSON single values within
the dialect-general test suite.

Fixes: #5014
Change-Id: Id38221dce1271fec527ca198b23908547b25d8a0

Added support for use of the :class:`.Sequence` construct with MariaDB 10.3
and greater, as this is now supported by this database.  The construct
integrates with the :class:`.Table` object in the same way that it does for
other databases like PostrgreSQL and Oracle; if is present on the integer
primary key "autoincrement" column, it is used to generate defaults.   For
backwards compatibility, to support a :class:`.Table` that has a
:class:`.Sequence` on it to support sequence only databases like Oracle,
while still not having the sequence fire off for MariaDB, the optional=True
flag should be set, which indicates the sequence should only be used to
generate the primary key if the target database offers no other option.

Fixes: #4976
Closes: #4996
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4996
Pull-request-sha: cb2e1426ea0b6bc6c93dbe8f033a11df9d8c4915

Change-Id: I507bc405eee6cae2c5991345d0eac53a37fe7512

Fixed the base class of the :class:`.mssql.DATETIMEOFFSET` datatype to
be based on the :class:`.DateTime` class hierarchy, as this is a
datetime-holding datatype.

Note however that at the moment the pyodbc driver has only limited
support for this datatype and will not work with all ODBC drivers.

References: https://github.com/mkleehammer/pyodbc/issues/134

Fixes: #4980
Change-Id: I946cf22642eea4beee2a605d91218c6fd817740c

The :class:`.oracle.INTERVAL` class of the Oracle dialect is now correctly
a subclass of the abstract version of :class:`.Interval` as well as the
correct "emulated" base class, which allows for correct behavior under both
native and non-native modes; previously it was only based on
:class:`.TypeEngine`.

Fixes: #4971
Change-Id: I4400d9f090330388460cca930e4139e3bd21eb11

Added "Connection was killed" message interpreted from the base
pymysql.Error class in order to detect closed connection, based on reports
that this message is arriving via a pymysql.InternalError() object which
indicates pymysql is not handling it correctly.

Change-Id: If6bbe0eb5993e1996c0c5de752eebaf7446cf93e
References: https://github.com/PyMySQL/PyMySQL/issues/816
Fixes: #4945

Added new accessors to expressions of type :class:`.JSON` to allow for
specific datatype access and comparison, covering strings, integers,
numeric, boolean elements.   This revises the documented approach of
CASTing to string when comparing values, instead adding specific
functionality into the PostgreSQL, SQlite, MySQL dialects to reliably
deliver these basic types in all cases.

The change also delivers a new feature to the test exclusions
system so that combinations and exclusions can be used together.

Fixes: #4276
Change-Id: Ica5a926c060feb40a0a7cd60b9d6e061d7825728