Remove the use of `six` from `unicode.py`.

Change-Id: I0f16debfbebf2a739a3cabf26700d2407b32b25c
Reviewed-on: https://code-review.googlesource.com/c/re2/+/60331
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Perry Lorier <perryl@google.com>
1 file changed