pg_readonly: psycopg3 support

[m1kc]

from django.db.backends.postgresql import base
class DatabaseWrapper(base.DatabaseWrapper):
	def get_new_connection(self, conn_params):
		conn = super(DatabaseWrapper, self).get_new_connection(conn_params)
		if hasattr(conn, 'read_only'):
			# pyscopg3
			conn.read_only = True
		elif hasattr(conn, 'set_session'):
			# pyscopg2
			conn.set_session(readonly=True)
		else:
			raise RuntimeError("pg_readonly: unknown adapter type (only pyscopg2 and pyscopg3 are supported)")
		return conn