`openpyxl`'s `ReadOnlyWorksheet.__getitem__()` cannot be used due to self type mismatching

[InSyncWithFoo]

Minimal reproducible example:

import openpyxl
from openpyxl.worksheet._read_only import ReadOnlyWorksheet

wb = openpyxl.load_workbook('...', read_only = True)
ws: ReadOnlyWorksheet = wb['foo']

ws[0]
# mypy => error:
# Invalid self argument "ReadOnlyWorksheet" to attribute function "__getitem__"
# with type "Callable[[Worksheet, int], tuple[Cell, ...]]"

The culprit is this assignment:

class ReadOnlyWorksheet:
    ...
    __getitem__ = Worksheet.__getitem__

(From this Stack Overflow question.)