Make StrPromise not inherit from Sequence[str] (#1841)

* Make `StrPromise` not inherit from `Sequence[str]`

* rm obsolete comment

django-stubs/utils/functional.pyi

@@ -1,4 +1,4 @@
-from collections.abc import Callable, Sequence
+from collections.abc import Callable
 
 # Mypy has special handling for functools.cached_property, reuse typeshed's definition instead of defining our own
 from functools import cached_property as cached_property
@@ -20,10 +20,9 @@ class Promise:
     def __radd__(self, other: Any) -> Any: ...
     def __deepcopy__(self, memo: Any) -> Self: ...
 
-class _StrPromise(Promise, Sequence[str]):
+class _StrPromise(Promise):
     def __add__(self, __s: str) -> str: ...
-    # Incompatible with Sequence.__contains__
-    def __contains__(self, __o: str) -> bool: ...  # type: ignore[override]
+    def __contains__(self, __o: str) -> bool: ...
     def __ge__(self, __x: str) -> bool: ...
     def __getitem__(self, __i: SupportsIndex | slice) -> str: ...
     def __gt__(self, __x: str) -> bool: ...