postgresql server version as a static variable

[findepi]

In django.db.backends.postgresql.base and django.db.backends.postgresql_psycopg2.base modules there is a code:

cursor = self.connection.cursor()
if set_tz:
    cursor.execute("SET TIME ZONE %s", [settings_dict['TIME_ZONE']])
    if not hasattr(self, '_version'):
        self.__class__._version = get_version(cursor)
    if self._version[0:2] < (8, 0):

It means that the server version is checked only at the first connection. I understand that django does not support and maybe will never support multiple different db connections from one process, but this may brake things even in case of database failover
E.g. at one network address there may be postgresql servers of different version available.

[Ramiro Morales]

In [16439]:

Fixed #11065, #11285 -- Streamlined PostgreSQL version detection, fixing incompatibility with multi-db. Thanks findepi for the report.

Changed our internal representation of the PostgreSQL version from tuples to
integers as used by libpq and psycopg2. This simplifies version comparison
operations.

Also, using the associated libpq/psycopg2 API allows to remove the need for
manually issuing in-band SELECT version() SQL queries to obtain the server
version (or at least reduce its number if version of psycopg2 in use is older
than 2.0.12). Refs #10509.