Fix signals problem on sentry.io

[antonpirker]

When using the newest version of the Python SDK on the sentry backend we get the following error:

tests/getsentry/billing/test_staged.py:838: in test_apply_inplace_trigger_post_save
    subscription = self.create_subscription(
    ...
/usr/local/lib/python3.8/site-packages/sentry_sdk/integrations/django/signals_handlers.py:56: in wrapper
    signal_name = _get_receiver_name(receiver)
/usr/local/lib/python3.8/site-packages/sentry_sdk/integrations/django/signals_handlers.py:28: in _get_receiver_name
    name = "partial(<function " + receiver.func.__name__ + ">)"  # type: ignore
/usr/local/lib/python3.8/unittest/mock.py:639: in __getattr__
    raise AttributeError(name)
E   AttributeError: __name__

This change gets the __name__ attribute in a very defensive way, to not raise any errors what so ever.

[vercel]

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Name	Status	Preview	Updated
sentry-python	✅ Ready (Inspect)	Visit Preview	Nov 10, 2022 at 1:33PM (UTC)

[sl0thentr0py]

please describe the problem that Tony reported in the PR description