python: Fix immediate test failures with PyPy

Update the newly added tests to account for variables missing
in `sysconfig` vars in PyPy.  Update the Python logic to handle missing
`c_api.pkgconfig_path`.

The logic is actually still buggy, as it will fall back to using
e.g. `python-3.11.pc` for PyPy3.11, and therefore building PyPy
extensions against CPython headers.  I've asked for suggestions
how to best deal with it in:
https://github.com/mesonbuild/meson/pull/13945/files#r2114254979

Author: mgorny

mesonbuild/dependencies/python.py

@@ -449,11 +449,13 @@ def __init__(self, environment: 'Environment', kwargs: T.Dict[str, T.Any],
         pkg_name = f'python-{installation.version}{pkg_embed}'
 
         if installation.build_config:
-            pkg_libdir = installation.build_config['c_api']['pkgconfig_path']
+            pkg_libdir = installation.build_config.get('c_api.pkgconfig_path')
             pkg_libdir_origin = 'c_api.pkgconfig_path from the Python build config'
         else:
             pkg_libdir = installation.info['variables'].get('LIBPC')
-            pkg_libdir_origin = 'LIBPC' if pkg_libdir else 'the default paths'
+            pkg_libdir_origin = 'LIBPC'
+        if pkg_libdir is None:
+            pkg_libdir_origin = 'the default paths'
         mlog.debug(f'Searching for {pkg_libdir!r} via pkgconfig lookup in {pkg_libdir_origin}')
         pkgconfig_paths = [pkg_libdir] if pkg_libdir else []
 

unittests/allplatformstests.py

@@ -3097,18 +3097,23 @@ def test_python_build_config_extensions(self):
             },
             'libpython': {
                 'dynamic': sysconfig.get_config_var('LDLIBRARY'),
-                'dynamic_stableabi': sysconfig.get_config_var('PY3LIBRARY'),
                 'static': sysconfig.get_config_var('LIBRARY'),
                 'link_extensions': True,
             },
             'c_api': {
                 'headers': sysconfig.get_path('include'),
-                'pkgconfig_path': sysconfig.get_config_var('LIBPC'),
             }
         }
+
+        py3library = sysconfig.get_config_var('PY3LIBRARY')
+        if py3library is not None:
+            python_build_config['libpython']['dynamic_stableabi'] = py3library
+        libpc = sysconfig.get_config_var('LIBPC')
+        if libpc is not None:
+            python_build_config['c_api']['pkgconfig_path'] = libpc
+
         with tempfile.NamedTemporaryFile(mode='w', delete=False, encoding='utf-8') as python_build_config_file:
             json.dump(python_build_config, fp=python_build_config_file)
-
         with tempfile.NamedTemporaryFile(mode='w', delete=False, encoding='utf-8') as cross_file:
             cross_file.write(
                 textwrap.dedent(f'''