Improve the interface used by adapted drivers, like the asyncio ones,
to access the actual connection object returned by the driver.

The :class:`_engine._ConnectionRecord` and
:class:`_engine._ConnectionFairy` now have two new attributes:

* ``dbapi_connection`` always represents a DBAPI compatible
object.  For pep-249 drivers, this is the DBAPI connection as it always
has been, previously accessed under the ``.connection`` attribute.
For asyncio drivers that SQLAlchemy adapts into a pep-249 interface,
the returned object will normally be a SQLAlchemy adaption object
called :class:`_engine.AdaptedConnection`.
* ``driver_connection`` always represents the actual connection object
maintained by the third party pep-249 DBAPI or async driver in use.
For standard pep-249 DBAPIs, this will always be the same object
as that of the ``dbapi_connection``.  For an asyncio driver, it will be
the underlying asyncio-only connection object.

The ``.connection`` attribute remains available and is now a legacy alias
of ``.dbapi_connection``.

Fixes: #6832
Change-Id: Ib72f97deefca96dce4e61e7c38ba430068d6a82e

Clarify Foreign Key support on SQLite must be enabled before `MetaData.create_all`

Change-Id: Ic41b86f736be21dd6fc890a915a2ffd572df73a4

Fixed bug where the error message for SQLite invalid isolation level on the
pysqlite driver would fail to indicate that "AUTOCOMMIT" is one of the
valid isolation levels.

Change-Id: Icbceab9a28af6a560859761fa92320b5473269a9
References: #6959

Also replace http://pypi.python.org/pypi with https://pypi.org/project

Change-Id: I84b5005c39969a82140706472989f2a30b0c7685

add additional notes regarding the methods that are on this
object.

Fixes: #6629
Change-Id: I68446cb16cbfcef9af9c2998687b3b536607cf0d

passed in the url in pysqlcipher.

Fixes: #6589
Change-Id: I86f93f84ef2bd374c4832a70e26e4901d024ed4b

The fix for pysqlcipher released in version 1.4.3 :ticket:`5848` was
unfortunately non-working, in that the new ``on_connect_url`` hook was
erroneously not receiving a ``URL`` object under normal usage of
:func:`_sa.create_engine` and instead received a string that was unhandled;
the test suite failed to fully set up the actual conditions under which
this hook is called. This has been fixed.

Fixes: #6586
Change-Id: I3bf738daec35877a10fdad740f08dca9e7420829

The :class:`.TypeDecorator` class will now emit a warning when used in SQL
compilation with caching unless the ``.cache_ok`` flag is set to ``True``
or ``False``. ``.cache_ok`` indicates that all the parameters passed to the
object are safe to be used as a cache key, ``False`` means they are not.

Fixes: #6436
Change-Id: Ib1bb7dc4b124e38521d615c2e2e691e4915594fb