Fix reachability for frozenset and dict view narrowing (#21151)

Fixes #21143 (on the same lines as #20704, i.e. there still exists a
deeper fix that could be made)

I also update a comment that is a little out of date since #20863 and
code in checkexpr.py that is out of date for modern typeshed

Author: hauntsaninja

mypy/checker.py

@@ -8734,7 +8734,13 @@ def reduce_and_conditional_type_maps(ms: list[TypeMap], *, use_meet: bool) -> Ty
     return result
 
 
-BUILTINS_CUSTOM_EQ_CHECKS: Final = {"builtins.bytearray", "builtins.memoryview"}
+BUILTINS_CUSTOM_EQ_CHECKS: Final = {
+    "builtins.bytearray",
+    "builtins.memoryview",
+    "builtins.frozenset",
+    "_collections_abc.dict_keys",
+    "_collections_abc.dict_items",
+}
 
 
 def has_custom_eq_checks(t: Type) -> bool:
@@ -8744,7 +8750,7 @@ def has_custom_eq_checks(t: Type) -> bool:
         # custom_special_method has special casing for builtins.* and typing.* that make the
         # above always return False. So here we return True if the a value of a builtin type
         # will ever compare equal to value of another type, e.g. a bytes value can compare equal
-        # to a bytearray value. We also include builtins collections, see testNarrowingCollections
+        # to a bytearray value.
         or (
             isinstance(pt := get_proper_type(t), Instance)
             and pt.type.fullname in BUILTINS_CUSTOM_EQ_CHECKS

mypy/checkexpr.py

@@ -236,8 +236,6 @@
     "builtins.frozenset",
     "typing.KeysView",
     "typing.ItemsView",
-    "builtins._dict_keys",
-    "builtins._dict_items",
     "_collections_abc.dict_keys",
     "_collections_abc.dict_items",
 ]