python: Use correct extension filename suffix on Python < 3.8.7

On Windows, in Python version prior to 3.8.7, the `sysconfig` modules
provides an extension filename suffix that disagrees the one returned
by `distutils.sysconfig`. Get the more awesome suffix from the latter
when building for a Python version known to present this issue.

Simplify the extension module filename suffix lookup to use the same
method used by `setuptools`.

Adjust project tests accordingly.

Fixes #10960.

mesonbuild/modules/python.py

@@ -372,6 +372,15 @@ def links_against_libpython():
 variables = sysconfig.get_config_vars()
 variables.update({'base_prefix': getattr(sys, 'base_prefix', sys.prefix)})
 
+if sys.version_info < (3, 0):
+    suffix = variables.get('SO')
+elif sys.version_info < (3, 8, 7):
+    # https://bugs.python.org/issue?@action=redirect&bpo=39825
+    from distutils.sysconfig import get_config_var
+    suffix = get_config_var('EXT_SUFFIX')
+else:
+    suffix = variables.get('EXT_SUFFIX')
+
 print(json.dumps({
   'variables': variables,
   'paths': paths,
@@ -382,6 +391,7 @@ print(json.dumps({
   'is_pypy': '__pypy__' in sys.builtin_module_names,
   'is_venv': sys.prefix != variables['base_prefix'],
   'link_libpython': links_against_libpython(),
+  'suffix': suffix,
 }))
 '''
 
@@ -440,8 +450,6 @@ class PythonExternalProgram(ExternalProgram):
             mlog.debug(stderr)
 
         if info is not None and self._check_version(info['version']):
-            variables = info['variables']
-            info['suffix'] = variables.get('EXT_SUFFIX') or variables.get('SO') or variables.get('.so')
             self.info = T.cast('PythonIntrospectionDict', info)
             self.platlib = self._get_path(state, 'platlib')
             self.purelib = self._get_path(state, 'purelib')

run_project_tests.py

@@ -177,7 +177,15 @@ class InstalledFile:
                 return p
             if self.typ == 'python_lib':
                 return p.with_suffix(python_suffix)
-        if self.typ in ['file', 'dir']:
+            if self.typ == 'py_implib':
+                p = p.with_suffix(python_suffix)
+                if env.machines.host.is_windows() and canonical_compiler == 'msvc':
+                    return p.with_suffix('.lib')
+                elif env.machines.host.is_windows() or env.machines.host.is_cygwin():
+                    return p.with_suffix('.dll.a')
+                else:
+                    return None
+        elif self.typ in {'file', 'dir'}:
             return p
         elif self.typ == 'shared_lib':
             if env.machines.host.is_windows() or env.machines.host.is_cygwin():
@@ -211,15 +219,13 @@ class InstalledFile:
             if self.version:
                 p = p.with_name('{}-{}'.format(p.name, self.version[0]))
             return p.with_suffix('.pdb') if has_pdb else None
-        elif self.typ in {'implib', 'implibempty', 'py_implib'}:
+        elif self.typ in {'implib', 'implibempty'}:
             if env.machines.host.is_windows() and canonical_compiler == 'msvc':
                 # only MSVC doesn't generate empty implibs
                 if self.typ == 'implibempty' and compiler == 'msvc':
                     return None
                 return p.parent / (re.sub(r'^lib', '', p.name) + '.lib')
             elif env.machines.host.is_windows() or env.machines.host.is_cygwin():
-                if self.typ == 'py_implib':
-                    p = p.with_suffix(python_suffix)
                 return p.with_suffix('.dll.a')
             else:
                 return None