Avoid creating ref cycles

By storing previously raised exceptions inside a local, this code created ref cycles that kept all locals in all calling stack frames alive.

This is because exceptions hold references to their tracebacks, which hold references to the relevant frames, which holds a reference to the local errors dict that holds references to the exceptions. See https://peps.python.org/pep-0344/#open-issue-garbage-collection and https://peps.python.org/pep-3110/#rationale

This breaks the cycle by deleting the local when we raise, so frames are destroyed by the normal reference counting mechanism.

This fixes some resource exhaustion issues I encountered at work.

src/typeguard/_checkers.py

@@ -405,7 +405,10 @@ def check_union(
     formatted_errors = indent(
         "\n".join(f"{key}: {error}" for key, error in errors.items()), "  "
     )
-    raise TypeCheckError(f"did not match any element in the union:\n{formatted_errors}")
+    try:
+        raise TypeCheckError(f"did not match any element in the union:\n{formatted_errors}")
+    finally:
+        del errors
 
 
 def check_uniontype(