Made some adjustments to :class:`.Pool` and :class:`.Connection` such
that recovery logic is not run underneath exception catches for
``pool.Empty``, ``AttributeError``, since when the recovery operation
itself fails, Python 3 creates a misleading stack trace referring to the
``Empty`` / ``AttributeError`` as the cause, when in fact these exception
catches are part of control flow.

Change-Id: Id3ed9a8f96ce4ccb4009c94af30ddc2ddb9818b9
Fixes: #4028

Added native "pessimistic disconnection" handling to the :class:`.Pool`
object.  The new parameter :paramref:`.Pool.pre_ping`, available from
the engine as :paramref:`.create_engine.pool_pre_ping`, applies an
efficient form of the "pre-ping" recipe featured in the pooling
documentation, which upon each connection check out, emits a simple
statement, typically "SELECT 1", to test the connection for liveness.
If the existing connection is no longer able to respond to commands,
the connection is transparently recycled, and all other connections
made prior to the current timestamp are invalidated.

Change-Id: I89700d0075e60abd2250e54b9bd14daf03c71c00
Fixes: #3919

Corrects some warnings and adds tox config.  Adds DeprecationWarning
to the error category.   Large sweep for string literals w/ backslashes
as this is common in docstrings

Co-authored-by: Andrii Soldatenko
Fixes: #3886
Change-Id: Ia7c838dfbbe70b262622ed0803d581edc736e085
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/337

Change-Id: I4e8c2aa8fe817bb2af8707410fa0201f938781de

Tests illustrate that exceptions like GreenletExit and
even KeyboardInterrupt can corrupt the state of a DBAPI
connection like that of pymysql and mysqlclient.   Intercept
BaseException errors within the handle_error scheme and
invalidate just the connection alone in this case, but not
the whole pool.

The change is backwards-incompatible with a program that
currently intercepts ctrl-C within a database transaction
and wants to continue working on that transaction.  Ensure
the event hook can be used to reverse this behavior.

Change-Id: Ifaa013c13826d123eef34e32b7e79fff74f1b21b
Fixes: #3803

- add a connect=True key to connection record to support
  pre-loading of _ConnectionRecord objects
- ensure _ConnectionRecord.close() leaves the record in a good
  state for reopening
- add _ConnectionRecord.record_info for persistent storage
- add "in_use" accessor based on fairy_ref being present or not
- allow for the exclusions system and SuiteRequirements to be
  usable without the full plugin_base setup.
- move some Python-env requirements to the importable
  requirements.py module.
- allow starttime to be queried
- add additional events for engine plugins
- have "dialect" be a first-class parameter to the pool,
  ensure the engine strategy supplies it up front

Change-Id: Ibf549f7a1766e49d335cd6f5e26bacfaef9a8229

has no return value

:meth:`.ConnectionEvents.detach`,
:meth:`.ConnectionEvents.close_detached`.