Fix python.lang.correctness.baseclass-attribute-override (#3434)

The rule depended on Semgrep unifying two identifiers, corresponding to
methods in two different classes, that were not the same but just had
the same "string" name.

python/lang/correctness/baseclass-attribute-override.yaml

@@ -9,17 +9,19 @@ rules:
       - pattern-inside: |
           class $A(...):
             ...
-            def $F(...):
+            def $F1(...):
               ...
             ...
           ...
       - pattern-inside: |
           class $B(...):
             ...
-            def $F(...):
+            def $F2(...):
               ...
             ...
           ...
+      - metavariable-comparison:
+          comparison: str($F1) == str($F2)
       - pattern: |
           class $C(..., $A, ..., $B, ...):
             ...