Disallow non-native psycopg2 Unicode in Python 3; update docs

Disallow non-native psycopg2 Unicode in Python 3; update docs

Fixed issue where the psycopg2 dialect would silently pass the
``use_native_unicode=False`` flag without actually having any effect under
Python 3, as the psycopg2 DBAPI uses Unicode unconditionally under Python
3.  This usage now raises an :class:`_exc.ArgumentError` when used under
Python 3. Added test support for Python 2.

Additionally, added documentation for client_encoding parameter
that may be passed to libpq directly via psycopg2.

Change-Id: I40ddf6382c157fa9399c21f0e01064197ea100f8