_handle_dbapi_exception_noconnection() to only invoke in the case
of raw_connection() in the constructor of Connection.   in all other
cases the Connection proceeds with _handle_dbapi_exception() including
revalidate.

to it, so that we say we will do the wrapping just once right here
in _execute_context() / _execute_default().  An adjustment is made
to _handle_dbapi_error() to not assume self.__connection in case
we are already in an invalidated state

further adjustment to
0639c199a547343d62134d2f233225fd2862ec45, 41e7253dee168b8c26c49, #3266

_handle_dbapi_error(); these are now handled already and the reentrant
call is not needed / breaks things.  Adjustment to 41e7253dee168b8c26c49 /

take effect in all engine connection use cases, including
when user-custom connect routines are used via the
:paramref:`.create_engine.creator` parameter, as well as when
the :class:`.Connection` encounters a connection error on
revalidation.
fixes #3266

Patch courtesy Gabor Gombas.
fixes #3127

for psycopg2 and others, encourage users to take advantage of positional
styles by documenting "paramstyle".   A section is added to psycopg2
specifically as this is a pretty common spot for named parameters
that may be unusually named.  fixes #3246.

that union() can be called, in the case of a dialect that uses
execution options inside of initialize() (e.g. oursql)

:paramref:`.create_engine.execution_options` or
:meth:`.Engine.update_execution_options` are not passed to the
special :class:`.Connection` used to initialize the dialect
within the "first connect" event; dialects will usually
perform their own queries in this phase, and none of the
current available  options should be applied here.  In
particular, the "autocommit" option was causing an attempt to
autocommit within this initial connect which would fail with
an AttributeError due to the non-standard state of the
:class:`.Connection`.
fixes #3200