Use more precise type for formparser.MultiPartParser.parse return

Without this change, `pyright` with `typeCheckingMode` set to `strict` reports:

```
error: Type of "fields" is partially unknown
    Type of "fields" is "MultiDict[Unknown, Unknown]" (reportUnknownVariableType)
```

when I use this method.

CHANGES.rst

@@ -1,5 +1,10 @@
 .. currentmodule:: werkzeug
 
+Version 3.0.2
+-------------
+
+-   Make the return type of ``MultiPartParser.parse`` more precise. :issue:`2840`
+
 Version 3.0.1
 -------------
 

src/werkzeug/formparser.py

@@ -352,7 +352,7 @@ def start_file_streaming(
 
     def parse(
         self, stream: t.IO[bytes], boundary: bytes, content_length: int | None
-    ) -> tuple[MultiDict, MultiDict]:
+    ) -> tuple[MultiDict[str, str], MultiDict[str, FileStorage]]:
         current_part: Field | File
         container: t.IO[bytes] | list[bytes]
         _write: t.Callable[[bytes], t.Any]