synapse.storage.engines: use psycopg2cffi compat hook

Plainly importing psycopg2cffi as psycopg2 is inadequate, because
psycopg2 or its components (psycopg2.extras) are being also imported
elsewhere and patching them all is prone to breakage.

Fixes #5054.

Signed-off-by: Ivan Shapovalov <intelfx@intelfx.name>

changelog.d/9270.misc

@@ -0,0 +1 @@
+Restore PyPy compatibility by using psycopg2cffi consistently.

synapse/storage/engines/__init__.py

@@ -30,9 +30,11 @@ def create_engine(database_config) -> BaseDatabaseEngine:
     if name == "psycopg2":
         # pypy requires psycopg2cffi rather than psycopg2
         if platform.python_implementation() == "PyPy":
-            import psycopg2cffi as psycopg2  # type: ignore
-        else:
-            import psycopg2  # type: ignore
+            from psycopg2cffi import compat  # type: ignore
+
+            compat.register()
+
+        import psycopg2  # type: ignore
 
         return PostgresEngine(psycopg2, database_config)