Misc typing fixes for tests, part 1 of N (#11323)

* Annotate HomeserverTestCase.servlets
* Correct annotation of federation_auth_origin
* Use AnyStr custom_headers instead of a Union

This allows (str, str) and (bytes, bytes).
This disallows (str, bytes) and (bytes, str)

* DomainSpecificString.SIGIL is a ClassVar

tests/unittest.py

@@ -20,7 +20,20 @@ import inspect
 import logging
 import secrets
 import time
-from typing import Any, Callable, Dict, Iterable, Optional, Tuple, Type, TypeVar, Union
+from typing import (
+    Any,
+    AnyStr,
+    Callable,
+    ClassVar,
+    Dict,
+    Iterable,
+    List,
+    Optional,
+    Tuple,
+    Type,
+    TypeVar,
+    Union,
+)
 from unittest.mock import Mock, patch
 
 from canonicaljson import json
@@ -45,6 +58,7 @@ from synapse.logging.context import (
     current_context,
     set_current_context,
 )
+from synapse.rest import RegisterServletsFunc
 from synapse.server import HomeServer
 from synapse.types import JsonDict, UserID, create_requester
 from synapse.util import Clock
@@ -204,15 +218,15 @@ class HomeserverTestCase(TestCase):
       config dict.
 
     Attributes:
-        servlets (list[function]): List of servlet registration function.
+        servlets: List of servlet registration function.
         user_id (str): The user ID to assume if auth is hijacked.
         hijack_auth (bool): Whether to hijack auth to return the user specified
         in user_id.
     """
 
-    servlets = []
     hijack_auth = True
     needs_threadpool = False
+    servlets: ClassVar[List[RegisterServletsFunc]] = []
 
     def __init__(self, methodName, *args, **kwargs):
         super().__init__(methodName, *args, **kwargs)
@@ -405,12 +419,10 @@ class HomeserverTestCase(TestCase):
         access_token: Optional[str] = None,
         request: Type[T] = SynapseRequest,
         shorthand: bool = True,
-        federation_auth_origin: str = None,
+        federation_auth_origin: Optional[bytes] = None,
         content_is_form: bool = False,
         await_result: bool = True,
-        custom_headers: Optional[
-            Iterable[Tuple[Union[bytes, str], Union[bytes, str]]]
-        ] = None,
+        custom_headers: Optional[Iterable[Tuple[AnyStr, AnyStr]]] = None,
         client_ip: str = "127.0.0.1",
     ) -> FakeChannel:
         """
@@ -425,7 +437,7 @@ class HomeserverTestCase(TestCase):
             a dict.
             shorthand: Whether to try and be helpful and prefix the given URL
             with the usual REST API path, if it doesn't contain it.
-            federation_auth_origin (bytes|None): if set to not-None, we will add a fake
+            federation_auth_origin: if set to not-None, we will add a fake
                 Authorization header pretenting to be the given server name.
             content_is_form: Whether the content is URL encoded form data. Adds the
                 'Content-Type': 'application/x-www-form-urlencoded' header.
@@ -639,9 +651,7 @@ class HomeserverTestCase(TestCase):
         username,
         password,
         device_id=None,
-        custom_headers: Optional[
-            Iterable[Tuple[Union[bytes, str], Union[bytes, str]]]
-        ] = None,
+        custom_headers: Optional[Iterable[Tuple[AnyStr, AnyStr]]] = None,
     ):
         """
         Log in a user, and get an access token. Requires the Login API be