TypeCodersTest is never executed

TypecodersTest relies on utils.check_compiled('apache_beam.coders') to skip if cython is not installed. But apache_beam.coders is never compiled (apache_beam.coders.coder_impl is) so this test is never executed.

https://github.com/apache/beam/blob/62904f469f4290546802e03189e5a8faa9af5b7d/sdks/python/apache_beam/coders/typecoders_test.py#L58