Remove an now-unused import in the tokenizer.

Two still fail because templates aren't implemented yet, but those
are otherwise handled correctly.

All tag tests passing. Still need to finish backslash support and
support for templates and tags within <open> tags.

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

Remove some extraneous whitespace in string_mixin.py.

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.