[3.13] gh-120449: fix `test_pyclbr` introspection for mangled names (GH-120450) (GH-120700)

gh-120449: fix ``test_pyclbr`` introspection for mangled names (GH-120450)
(cherry picked from commit d8cd0fa4e3)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Lib/test/pyclbr_input.py

@@ -12,17 +12,19 @@ class B (object):
     def bm(self): pass
 
 class C (B):
-    foo = Other().foo
-    om = Other.om
-
     d = 10
 
-    # XXX: This causes test_pyclbr.py to fail, but only because the
-    #      introspection-based is_method() code in the test can't
-    #      distinguish between this and a genuine method function like m().
-    #      The pyclbr.py module gets this right as it parses the text.
+    # This one is correctly considered by both test_pyclbr.py and pyclbr.py
+    # as a non-method of C.
+    foo = Other().foo
+
+    # This causes test_pyclbr.py to fail, but only because the
+    # introspection-based is_method() code in the test can't
+    # distinguish between this and a genuine method function like m().
     #
-    #f = f
+    # The pyclbr.py module gets this right as it parses the text.
+    om = Other.om
+    f = f
 
     def m(self): pass
 
@@ -31,3 +33,53 @@ class C (B):
 
     @classmethod
     def cm(self): pass
+
+# Check that mangling is correctly handled
+
+class a:
+    def a(self): pass
+    def _(self): pass
+    def _a(self): pass
+    def __(self): pass
+    def ___(self): pass
+    def __a(self): pass
+
+class _:
+    def a(self): pass
+    def _(self): pass
+    def _a(self): pass
+    def __(self): pass
+    def ___(self): pass
+    def __a(self): pass
+
+class __:
+    def a(self): pass
+    def _(self): pass
+    def _a(self): pass
+    def __(self): pass
+    def ___(self): pass
+    def __a(self): pass
+
+class ___:
+    def a(self): pass
+    def _(self): pass
+    def _a(self): pass
+    def __(self): pass
+    def ___(self): pass
+    def __a(self): pass
+
+class _a:
+    def a(self): pass
+    def _(self): pass
+    def _a(self): pass
+    def __(self): pass
+    def ___(self): pass
+    def __a(self): pass
+
+class __a:
+    def a(self): pass
+    def _(self): pass
+    def _a(self): pass
+    def __(self): pass
+    def ___(self): pass
+    def __a(self): pass