[PR #8211/7725f5a2 backport][3.10] Fix type annotations on MultipartW…

…riter.append (#8215)

**This is a backport of PR #8211 as merged into master
(7725f5a).**

Co-authored-by: Daniel Golding <goldingd89@gmail.com>

CHANGES/7741.bugfix.rst

@@ -0,0 +1,3 @@
+Changed the type annotations to allow ``dict`` on :meth:`aiohttp.MultipartWriter.append`,
+:meth:`aiohttp.MultipartWriter.append_json` and
+:meth:`aiohttp.MultipartWriter.append_form` -- by :user:`cakemanny`

CONTRIBUTORS.txt

@@ -80,6 +80,7 @@ Damien Nadé
 Dan King
 Dan Xu
 Daniel García
+Daniel Golding
 Daniel Grossmann-Kavanagh
 Daniel Nelson
 Danny Song

aiohttp/multipart.py

@@ -25,7 +25,7 @@
 )
 from urllib.parse import parse_qsl, unquote, urlencode
 
-from multidict import CIMultiDict, CIMultiDictProxy, MultiMapping
+from multidict import CIMultiDict, CIMultiDictProxy
 
 from .compression_utils import ZLibCompressor, ZLibDecompressor
 from .hdrs import (
@@ -791,7 +791,7 @@ def _boundary_value(self) -> str:
     def boundary(self) -> str:
         return self._boundary.decode("ascii")
 
-    def append(self, obj: Any, headers: Optional[MultiMapping[str]] = None) -> Payload:
+    def append(self, obj: Any, headers: Optional[Mapping[str, str]] = None) -> Payload:
         if headers is None:
             headers = CIMultiDict()
 
@@ -839,7 +839,7 @@ def append_payload(self, payload: Payload) -> Payload:
         return payload
 
     def append_json(
-        self, obj: Any, headers: Optional[MultiMapping[str]] = None
+        self, obj: Any, headers: Optional[Mapping[str, str]] = None
     ) -> Payload:
         """Helper to append JSON part."""
         if headers is None:
@@ -850,7 +850,7 @@ def append_json(
     def append_form(
         self,
         obj: Union[Sequence[Tuple[str, str]], Mapping[str, str]],
-        headers: Optional[MultiMapping[str]] = None,
+        headers: Optional[Mapping[str, str]] = None,
     ) -> Payload:
         """Helper to append form urlencoded part."""
         assert isinstance(obj, (Sequence, Mapping))