refactor: Use a custom class instead of object for sentinel instance

It makes typing stricter, since types don't depend on a very generic `object` type

src/schemathesis/types.py

@@ -13,7 +13,12 @@
 Cookies = Dict[str, Any]  # pragma: no mutate
 FormData = Dict[str, Any]  # pragma: no mutate
 
+
+class NotSet:
+    pass
+
+
 # A filter for endpoint / method
-Filter = Union[str, List[str], Tuple[str], Set[str], object]  # pragma: no mutate
+Filter = Union[str, List[str], Tuple[str], Set[str], NotSet]  # pragma: no mutate
 
 Hook = Callable[[SearchStrategy], SearchStrategy]

src/schemathesis/utils.py

@@ -11,9 +11,9 @@
 from werkzeug.wrappers import Response as BaseResponse
 from werkzeug.wrappers.json import JSONMixin
 
-from .types import Filter
+from .types import Filter, NotSet
 
-NOT_SET = object()
+NOT_SET = NotSet()
 
 
 def deprecated(func: Callable, message: str) -> Callable: