* Don't mutate the standard library token.tok_name dictionary
Fixes#41.
* More robust test that tok_name isn't mutated
This test now works in Python 2.7, and actually tests something in Python 3.7,
and it's better anyway because it tests the whole dictionary instead of just
one token.
* Fix test_tok_name_copied in Python 3.7 and PyPy
Apparently Python 3.7 adds N_TOKENS to the tok_name dictionary, and PyPy
doesn't have NT_OFFSET in it.