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.

Conflicts:
	tests/test_builder.py

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

Conflicts:
	mwparserfromhell/parser/contexts.py
	mwparserfromhell/parser/tokenizer.py

Remove some extraneous whitespace in string_mixin.py.