Switch to PyPI publishing via a trusted publisher. Change-Id: Ie94c5e6931f8fa70f238c9a5360874ff8833876a Reviewed-on: https://code-review.googlesource.com/c/re2/+/62972 Reviewed-by: Perry Lorier <perryl@google.com> Reviewed-by: Paul Wankadia <junyer@google.com>