python: require Python 3.9+

The module has been building wheels for Python 3.9 and later,
but setup.py still declared a minimum of 3.8.
Update the metadata to require 3.9.

Fixes #559.

Change-Id: Id5d06a006f5080529b69bfe163bf629e36b15c34
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63850
Reviewed-by: Alan Donovan <adonovan@google.com>

python/setup.py

@@ -144,11 +144,11 @@ try:
           'Intended Audience :: Developers',
           'License :: OSI Approved :: BSD License',
           'Programming Language :: C++',
-          'Programming Language :: Python :: 3.8',
+          'Programming Language :: Python :: 3.9',
       ],
       options=options(),
       cmdclass={'build_ext': BuildExt},
-      python_requires='~=3.8',
+      python_requires='~=3.9',
   )
 except:
   raise