type_enforce test failure on 3.9

[thatch]

Once #297 is fixed, there are a couple of failures. This one is a behavior change in typing.Iterable.

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ERROR: test_basic_pass_19 (libcst.tests.test_type_enforce.TypeEnforcementTest)
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Traceback (most recent call last):
  File "/home/tim/code/LibCST/libcst/testing/utils.py", line 89, in new_test
    return member(self, *data)
  File "/home/tim/code/LibCST/libcst/tests/test_type_enforce.py", line 135, in test_basic_pass
    is_value_of_type(value, expected_type),
  File "/home/tim/code/LibCST/libcst/_type_enforce.py", line 136, in is_value_of_type
    elif issubclass(expected_origin_type, Iterable) and not issubclass(
TypeError: issubclass() arg 1 must be a class

It appears to be a difference in typing_inspect.get_origin.