Fix stubtest false positive for properties with a deleter (python#21259)

Previously, properties with a deleter but no setter were treated as
read-only.
Update logic to consider a property read-only only if both `fset` and
`fdel` are None.

Fixes python#21254

Author: pranavmanglik

mypy/stubtest.py

@@ -1851,7 +1851,7 @@ def is_probably_a_function(runtime: Any) -> bool:
 
 
 def is_read_only_property(runtime: object) -> bool:
-    return isinstance(runtime, property) and runtime.fset is None
+    return isinstance(runtime, property) and runtime.fset is None and runtime.fdel is None
 
 
 def safe_inspect_signature(runtime: Any) -> inspect.Signature | None:

mypy/test/teststubtest.py

@@ -1061,6 +1061,25 @@ class FineAndDandy:
             """,
             error=None,
         )
+        yield Case(
+            stub="""
+            class spam:
+                @property
+                def eggs(self) -> int: ...
+                @eggs.deleter
+                def eggs(self) -> None: ...
+            """,
+            runtime="""
+            class spam:
+                @property
+                def eggs(self):
+                    return 42
+                @eggs.deleter
+                def eggs(self):
+                    print("eggs")
+            """,
+            error=None,
+        )
 
     @collect_cases
     def test_cached_property(self) -> Iterator[Case]: