Remove an now-unused import in the tokenizer.

- Make a test in Py3k actually use StringMixIn instead of str.
- Minor cosmetic fix.

Also add one for arguments.

Also add a couple for templates and one for integration.

Mostly by @gdooms, with tweaks.

- Also add tests for spacing in param names.

- Stop parsing new templates if the template depth gets above
  MAX_DEPTH (40) or if we've already tried to parse over MAX_CYCLES
  (100,000) templates.
- Add two tests to ensure recursion works somewhat correctly.
- Fix parsing the string "{{" with the Python tokenizer; add a test.