Don't mutate the standard library token.tok_name dictionary (#42)

* 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.

parso/python/token.py

@@ -4,6 +4,8 @@ from token import *
 
 from parso._compatibility import py_version
 
+# Don't mutate the standard library dict
+tok_name = tok_name.copy()
 
 _counter = count(N_TOKENS)
 # Never want to see this thing again.

test/test_tokenize.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8    # This file contains Unicode characters.
 
 from textwrap import dedent
+import tokenize as stdlib_tokenize
 
 import pytest
 
@@ -235,3 +236,14 @@ def test_error_string():
     assert t1.prefix == ' '
     assert t1.string == '"\n'
     assert endmarker.string == ''
+
+def test_tok_name_copied():
+    # Make sure parso doesn't mutate the standard library
+    tok_len = len(stdlib_tokenize.tok_name)
+    correct_len = stdlib_tokenize.N_TOKENS
+    if 'N_TOKENS' in stdlib_tokenize.tok_name.values(): # Python 3.7
+        correct_len += 1
+    if 'NT_OFFSET' in stdlib_tokenize.tok_name.values(): # Not there in PyPy
+        correct_len += 1
+
+    assert tok_len == correct_len