Be less fiscal regarding validators functions

Be less fiscal regarding validators functions

Adjusted the check in the mapper for a callable object that is used as a
``@validates`` validator function or a ``@reconstructor`` reconstruction
function, to check for "callable" more liberally such as to accommodate
objects based on fundamental attributes like ``__func__`` and
``__call___``, rather than testing for ``MethodType`` / ``FunctionType``,
allowing things like cython functions to work properly. Pull request
courtesy Miłosz Stypiński.

Fixes: #6538
Closes: #6539
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/6539
Pull-request-sha: ed1d7fe5c9386bab0416ff32095afc777c26b6ca

Change-Id: I8350558bc9a9ba58f43e48e12ce25a0b30e4d767