Revert "Make the extension work for Python 3.8+."

This reverts commit 3a9939d2ae.

Reason for revert: pybind11 doesn't support `Py_LIMITED_API`;
as per https://github.com/pybind/pybind11/issues/1755, it has
probably never worked and it will probably never work.

Change-Id: Ie603151d31ad07a77d469f0050270d416fe8d21b
Reviewed-on: https://code-review.googlesource.com/c/re2/+/61872
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>

python/_re2.cc

@@ -2,10 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// The extension works for Python 3.8+.
-// See https://docs.python.org/3/c-api/stable.html for background.
-#define Py_LIMITED_API 0x03080000
-
 #include <memory>
 #include <string>
 #include <tuple>

python/setup.py

@@ -74,9 +74,6 @@ def options():
     bdist_wheel['plat_name'] = os.environ['PLAT_NAME']
   except KeyError:
     pass
-  # The extension works for Python 3.8+.
-  # See https://docs.python.org/3/c-api/stable.html for background.
-  bdist_wheel['py_limited_api'] = 'cp38'
   return {'bdist_wheel': bdist_wheel}