Codegen test failure on 3.9

[thatch]

Once #297 is fixed, there are a couple of failures. This one is that the codegen test fails, and indeed it would create a rather significant diff if you run codegen under 3.9.

https://gist.github.com/thatch/7f402910a46fdbe6c4e17a9493ef91a5

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FAIL: test_codegen_clean_matcher_classes (libcst.codegen.tests.test_codegen_clean.TestCodegenClean)
Verifies that codegen of matcher classes would not result in a
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Traceback (most recent call last):
  File "/home/tim/code/LibCST/libcst/codegen/tests/test_codegen_clean.py", line 82, in test_codegen_clean_matcher_classes
    self.assertTrue(
AssertionError: False is not true : libcst.matchers.__init__ needs new codegen!