Don't emit warnings on descriptor access

Don't emit warnings on descriptor access

This commit is revising 5162f2bc5fc0ac239f26a76fc9f0c2, which
when I did it felt a little rushed but I couldn't find anything
wrong.  Well here we are :).

Fixed issue where a :class:`.RemovedIn20Warning` would erroneously emit
when the ``.bind`` attribute were accessed internally on objects,
particularly when stringifying a SQL construct.

Alter the deprecated() decorator so that we can use it just to add
docstring warnings but not actually warn when the function is
accessed, adding new argument enable_warnings that can be
set to False.

Added a safety feature to deprecated_20() that will disallow an
":attr:" from proceeding if enable_warnings=False isn't present,
unless there's an extra flag
warn_on_attribute_access, since we want Session.transaction to
emit a deprecation warning.  This is a little hacky but it's essentially
modifying the decorator to require a positive assertion that
a deprecation decorator on a descriptor should actually warn
on access.

Remove the warning filter for session.transaction and get
tests to pass to ensure this is not also being called
internally.

Added tests to ensure that common places .bind can be passed
as a parameter definitely warn as I was not able to find
this otherwise.

Fixes: #5717
Change-Id: Ia586b4f9ee6b212f3a71104b1caf40b5edd399e2