setup.py: avoid build errors with PY_LIMITED_API and Python-3.13.

SWIG-generated code does not currently work with PY_LIMITED_API and
Python-3.13, so we disable PY_LIMITED_API by default.

This does not effect releases because they are all built with Python-3.9 and
PY_LIMITED_API. But this allows standalone builds to work.

Author: julian-smith-artifex-com

setup.py

@@ -152,6 +152,11 @@
     
     PYMUPDF_SETUP_PY_LIMITED_API
         If not '0', we build for current Python's stable ABI.
+        
+        However if unset and we are on Python-3.13 or later, we do
+        not build for the stable ABI because as of 2025-03-04 SWIG
+        generates incorrect stable ABI code with Python-3.13 - see:
+        https://github.com/swig/swig/issues/3059
     
     PYMUPDF_SETUP_URL_WHEEL
         If set, we use an existing wheel instead of building a new wheel.
@@ -242,10 +247,16 @@ def run(command, check=1):
 # Name of file that identifies that we are in a PyMuPDF sdist.
 g_pymupdfb_sdist_marker = 'pymupdfb_sdist'
 
+python_version_tuple = tuple(int(x) for x in platform.python_version_tuple()[:2])
+
 PYMUPDF_SETUP_PY_LIMITED_API = os.environ.get('PYMUPDF_SETUP_PY_LIMITED_API')
 assert PYMUPDF_SETUP_PY_LIMITED_API in (None, '', '0', '1'), \
         f'Should be "", "0", "1" or undefined: {PYMUPDF_SETUP_PY_LIMITED_API=}.'
-g_py_limited_api = (PYMUPDF_SETUP_PY_LIMITED_API != '0')
+if PYMUPDF_SETUP_PY_LIMITED_API is None and python_version_tuple >= (3, 13):
+    log(f'Not defaulting to Python limited api because {platform.python_version_tuple()=}.')
+    g_py_limited_api = False
+else:
+    g_py_limited_api = (PYMUPDF_SETUP_PY_LIMITED_API != '0')
 
 PYMUPDF_SETUP_URL_WHEEL =  os.environ.get('PYMUPDF_SETUP_URL_WHEEL')
 log(f'{PYMUPDF_SETUP_URL_WHEEL=}')