Type hint FakeAdapter and assertion function using a type protocol

src/requtests/fake_adapter.py

@@ -1,9 +1,24 @@
 from itertools import cycle
+from typing import Any, List, Union
+from typing_extensions import Protocol
 from requests.adapters import BaseAdapter
+from requests.models import Response, PreparedRequest
+
+
+class AssertionFunction(Protocol):
+    def __call__(self, prepared_request: PreparedRequest, **kwargs) -> Any:
+        """
+        An assertion function is expected to raise an error if any of its assertions fail.
+        """
+        pass
 
 
 class FakeAdapter(BaseAdapter):
-    def __init__(self, *responses, assertions=None):
+    def __init__(
+        self,
+        *responses: Response,
+        assertions: Union[None, AssertionFunction, List[AssertionFunction]] = None
+    ):
         super().__init__()
         self.closed = 0
         self.responses = _to_generator(responses)