Fix false-positive used-before-assignment in function returns

[cdce8p]

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Description

This MR addresses a false-positive that occurred for assignment expressions in function returns.
The following is valid and should not raise an used-before-assignment error.

def func():
    return f'The number {(count := 4)} ' \
           f'is equal to {count}'

This is a followup to #4253

Type of Changes

	Type
✓	🐛 Bug fix

Related Issue

Closes #4301

[coveralls]

Coverage remained the same at 91.593% when pulling 44b8e6c on cdce8p:fix-assignment-expressions into 0b250de on PyCQA:master.

[Pierre-Sassoulas]

👍