synapse.app.base: only call gc.freeze() on CPython

gc.freeze() is an implementation detail of CPython garbage collector,
and notably does not exist on PyPy.

Rather than playing whack-a-mole and skipping the call when under PyPy,
simply restrict it to CPython because the whole gc module is
implementation-defined.

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

changelog.d/9270.misc

@@ -1 +1 @@
-Restore PyPy compatibility by using psycopg2cffi consistently.
+Restore PyPy compatibility by using psycopg2cffi consistently and not calling CPython GC methods.

synapse/app/_base.py

@@ -16,6 +16,7 @@
 import gc
 import logging
 import os
+import platform
 import signal
 import socket
 import sys
@@ -339,7 +340,7 @@ def run_sighup(*args, **kwargs):
     # rest of time. Doing so means less work each GC (hopefully).
     #
     # This only works on Python 3.7
-    if sys.version_info >= (3, 7):
+    if platform.python_implementation() == "CPython" and sys.version_info >= (3, 7):
         gc.collect()
         gc.freeze()