gh-139276: Remove generator type check in _testcapimodule.c:raise_SIGINT_then_send_None (#139252)

* Remove generator type check in raise_SIGINT_then_send_None

In the Cinder JIT we use a different type for generators, which breaks
the test which uses this function.

In general I believe the intent with generators is they have the right
structure rather than type, so a failure to find the 'send()' method is arguably
more correct if the wrong object is used.

* Also stop using PyGenObject type