Use more precise type for formparser.MultiPartParser.parse return

[adamtheturtle]

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.

• fixes pyright in strict mode errors when using formparser.MultiPartParser.parse #2840

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[pgjones]

Thank you