[PR #8776/11171b8d backport][3.11] Use more precise headers type (#8778)

**This is a backport of PR #8776 as merged into master
(11171b8).**

---------

Co-authored-by: Sam Bull <git@sambull.org>

CHANGES/8768.bugfix.rst

@@ -0,0 +1 @@
+Used more precise type for ``ClientResponseError.headers``, fixing some type errors when using them -- by :user:`Dreamorcerer`.

aiohttp/client_exceptions.py

@@ -4,8 +4,10 @@
 import warnings
 from typing import TYPE_CHECKING, Optional, Tuple, Union
 
+from multidict import MultiMapping
+
 from .http_parser import RawResponseMessage
-from .typedefs import LooseHeaders, StrOrURL
+from .typedefs import StrOrURL
 
 try:
     import ssl
@@ -71,7 +73,7 @@ def __init__(
         code: Optional[int] = None,
         status: Optional[int] = None,
         message: str = "",
-        headers: Optional[LooseHeaders] = None,
+        headers: Optional[MultiMapping[str]] = None,
     ) -> None:
         self.request_info = request_info
         if code is not None:

tests/test_client_exceptions.py

@@ -5,6 +5,7 @@
 from unittest import mock
 
 import pytest
+from multidict import CIMultiDict
 from yarl import URL
 
 from aiohttp import client, client_reqrep
@@ -44,7 +45,7 @@ def test_pickle(self) -> None:
             history=(),
             status=400,
             message="Something wrong",
-            headers={},
+            headers=CIMultiDict(foo="bar"),
         )
         err.foo = "bar"
         for proto in range(pickle.HIGHEST_PROTOCOL + 1):
@@ -54,7 +55,8 @@ def test_pickle(self) -> None:
             assert err2.history == ()
             assert err2.status == 400
             assert err2.message == "Something wrong"
-            assert err2.headers == {}
+            # Use headers.get() to verify static type is correct.
+            assert err2.headers.get("foo") == "bar"
             assert err2.foo == "bar"
 
     def test_repr(self) -> None:
@@ -66,11 +68,11 @@ def test_repr(self) -> None:
             history=(),
             status=400,
             message="Something wrong",
-            headers={},
+            headers=CIMultiDict(),
         )
         assert repr(err) == (
             "ClientResponseError(%r, (), status=400, "
-            "message='Something wrong', headers={})" % (self.request_info,)
+            "message='Something wrong', headers=<CIMultiDict()>)" % (self.request_info,)
         )
 
     def test_str(self) -> None:
@@ -79,7 +81,7 @@ def test_str(self) -> None:
             history=(),
             status=400,
             message="Something wrong",
-            headers={},
+            headers=CIMultiDict(),
         )
         assert str(err) == (
             "400, message='Something wrong', " "url='http://example.com'"