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/rest/client/utils.py

@@ -19,7 +19,17 @@ import json
 import re
 import time
 import urllib.parse
-from typing import Any, Dict, Iterable, Mapping, MutableMapping, Optional, Tuple, Union
+from typing import (
+    Any,
+    AnyStr,
+    Dict,
+    Iterable,
+    Mapping,
+    MutableMapping,
+    Optional,
+    Tuple,
+    Union,
+)
 from unittest.mock import patch
 
 import attr
@@ -53,9 +63,7 @@ class RestHelper:
         tok: Optional[str] = None,
         expect_code: int = 200,
         extra_content: Optional[Dict] = None,
-        custom_headers: Optional[
-            Iterable[Tuple[Union[bytes, str], Union[bytes, str]]]
-        ] = None,
+        custom_headers: Optional[Iterable[Tuple[AnyStr, AnyStr]]] = None,
     ) -> str:
         """
         Create a room.
@@ -227,9 +235,7 @@ class RestHelper:
         txn_id=None,
         tok=None,
         expect_code=200,
-        custom_headers: Optional[
-            Iterable[Tuple[Union[bytes, str], Union[bytes, str]]]
-        ] = None,
+        custom_headers: Optional[Iterable[Tuple[AnyStr, AnyStr]]] = None,
     ):
         if body is None:
             body = "body_text_here"
@@ -418,7 +424,7 @@ class RestHelper:
             path,
             content=image_data,
             access_token=tok,
-            custom_headers=[(b"Content-Length", str(image_length))],
+            custom_headers=[("Content-Length", str(image_length))],
         )
 
         assert channel.code == expect_code, "Expected: %d, got: %d, resp: %r" % (