Reworked the proxy creation used by scoped_session() to be
based on fully copied code with augmented docstrings and
moved it into langhelpers.  asyncio session, engine,
connection can now take
advantage of it so that all non-async methods are availble.

Overall implementation of most important accessors / methods
on AsyncConnection, etc. , including awaitable versions
of invalidate, execution_options, etc.

In order to support an event dispatcher on the async
classes while still allowing them to hold __slots__,
make some adjustments to the event system to allow
that to be present, at least rudimentally.

Fixes: #5628
Change-Id: I5eb6929fc1e4fdac99e4b767dcfd49672d56e2b2

Fixes: #5634
Change-Id: Ie8d4076ee35234b535a04e6fb9321096df3f648b

This reverts commit 0220b58917b5a979891b5765f6ac5095e0368489.

I completely misread https://www.python.org/dev/peps/pep-0418/#rationale
and the accuracy of monotonic() is *worse* on windows than time.time(),
which is bizarre.

Change-Id: I2d571e268a2051bea68736507773d3904403af9e

The internal clock used by the :class:`_pool.Pool` object is now
time.monotonic_time() under Python 3.  Under Python 2, time.time() is still
used, which is legacy. This clock is used to measure the age of a
connection against its starttime, and used in comparisons against the
pool_timeout setting as well as the last time the pool was marked as
invalid to determine if the connection should be recycled. Previously,
time.time() was used which was subject to inaccuracies as a result of
system clock changes as well as poor time resolution on windows.

Change-Id: I94f90044c1809508e26a5a00134981c2a00d0405

Repaired a function-level import that was not using SQLAlchemy's standard
late-import system within the sqlalchemy.exc module.

Moved preloaded to sqlalchemy.util.preloaded so that it
does not depend on langhelpers which depends on exc.

Fixes: #5632
Change-Id: I61b7ce9cd461071ce543714739f67aa5aeb47fd6

Fixes: #5223
Change-Id: I0952e54ed9af2952ea340be1945311376ffc1ad2

The ORM Declarative system is now unified into the ORM itself, with new
import spaces under ``sqlalchemy.orm`` and new kinds of mappings.  Support
for decorator-based mappings without using a base class, support for
classical style-mapper() calls that have access to the declarative class
registry for relationships, and full integration of Declarative with 3rd
party class attribute systems like ``dataclasses`` and ``attrs`` is now
supported.

Fixes: #5508
Change-Id: I130b2b6edff6450bfe8a3e6baa099ff04b5471ff

Added support for PostgreSQL "readonly" and "deferrable" flags for all of
psycopg2, asyncpg and pg8000 dialects.   This takes advantage of a newly
generalized version of the "isolation level" API to support other kinds of
session attributes set via execution options that are reliably reset
when connections are returned to the connection pool.

Fixes: #5549
Change-Id: I0ad6d7a095e49d331618274c40ce75c76afdc7dd

Using the approach introduced at
https://gist.github.com/zzzeek/6287e28054d3baddc07fa21a7227904e

We can now create asyncio endpoints that are then handled
in "implicit IO" form within the majority of the Core internals.
Then coroutines are re-exposed at the point at which we call
into asyncpg methods.

Patch includes:

* asyncpg dialect

* asyncio package

* engine, result, ORM session classes

* new test fixtures, tests

* some work with pep-484 and a short plugin for the
  pyannotate package, which seems to have so-so results

Change-Id: Idbcc0eff72c4cad572914acdd6f40ddb1aef1a7d
Fixes: #3414