Fixes for PyPy compatibility (#9270)

* 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>