Add type inference for dict.keys membership

Also for containership checks on `typing.KeysView.` This is to bring the
following cases into alignment:

    from typing import KeysView

    key: str | None
    d: dict[str, int]
    kv: KeysView[str]

    if key in d:
        # type of 'key' is inferred to be 'str'
        ...

    if key in d.keys():
        # type of 'key' should be inferred to be 'str'
        ...

    if key in kv:
        # type of 'key' should be inferred to be 'str'
        ...

Before this change only the first `if` would narrow the type of `key`.

I've just added a test under `test-data/unit/pythoneval.test` as
`test-data/unit/fixtures/dict.pyi` doesn't include `dict.keys`, and
adding it requires importing `dict_keys` from `_collections_abc` in
those stubs, which then requires adding `_collections_abc.pyi` stubs,
which would have to be complete since e.g.
`testGenericAliasCollectionsABCReveal` expects most of the types in
those stubs to be defined (this is the same approach as
7678f28).

GH: issue #13360

mypy/checker.py

@@ -6329,6 +6329,8 @@ def builtin_item_type(tp: Type) -> Type | None:
             "builtins.dict",
             "builtins.set",
             "builtins.frozenset",
+            "_collections_abc.dict_keys",
+            "typing.KeysView",
         ]:
             if not tp.args:
                 # TODO: fix tuple in lib-stub/builtins.pyi (it should be generic).

test-data/unit/pythoneval.test

@@ -1636,3 +1636,27 @@ foo("")
 foo(list(""))
 foo(list((list(""), "")))
 [out]
+
+[case testNarrowTypeForDictKeys]
+# flags: --strict-optional
+from typing import Dict, KeysView, Optional
+
+d: Dict[str, int]
+key: Optional[str]
+if key in d.keys():
+    reveal_type(key)
+else:
+    reveal_type(key)
+
+kv: KeysView[str]
+k: Optional[str]
+if k in kv:
+    reveal_type(k)
+else:
+    reveal_type(k)
+
+[out]
+_testNarrowTypeForDictKeys.py:7: note: Revealed type is "builtins.str"
+_testNarrowTypeForDictKeys.py:9: note: Revealed type is "Union[builtins.str, None]"
+_testNarrowTypeForDictKeys.py:14: note: Revealed type is "builtins.str"
+_testNarrowTypeForDictKeys.py:16: note: Revealed type is "Union[builtins.str, None]"