The test_imp stuff has to do with PyTokenizer_FindEncoding().
imp.find_module() only opens the file, passes the file descriptor to
PyTokenizer_FindEncoding() and then returns a file object with the found
encoding.

Problem is that (as issue 3594 points out), PyTokenizer_FindEncoding()
always fails. That means it assumes only the raw encodings are okay.
With Latin-1 being one of them, it returns the file opened as Latin-1 as
is correct. Removing that case here means PyTokenizer_FindEncoding()
fails, and thus assumes only UTF-8 as a legitimate encoding and opens
the files with the UTF-8 encoding. It took a while to find these two
bugs obviously. =)