Effectively renamed tokenize_tests.py to have a txt extension instead.

Effectively renamed tokenize_tests.py to have a txt extension instead.
This file isn't meant to be executed, it's data input for test_tokenize.py.
The problem with the .py extension is that it uses "non-standard"
indentation, and it's good to test that, but reindent.py keeps wanting
to fix it.  But fixing the indentation causes the expected-output file to
change, since exact line and column numbers are part of the
tokenize.tokenize() output getting tested.