Tidy up the Python build a little.

Change-Id: I0b152cd0820df0e1e316dbc949e0548043a16600
Reviewed-on: https://code-review.googlesource.com/c/re2/+/61471
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>