fix(stubgen): extract unpack assignments

[tobyh-canva]

Context

Stubgen currently skips unpack assignments, for example:

A, B = 1, 2

doesn't emit anything in stubs.

It should emit something like:

from typing import Literal

A: Literal[1]
B: Literal[2]

Here's an example in the wild where unpack assignments are used to create public ClassVar constants: WordNetCorpusReader from nltk

Changes

• Handle unpack assignments in Stubgen's assignment extraction machinery.

Test Plan

• Added new stubgen snapshot test describing the desired behaviour

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅