openpyxl: Workbook opened in readonly mode should return `ReadOnlyCell | EmptyCell` from `worksheet.rows`

Currently the type hints for worksheet.rows returns Cell objects. However, opening a workbook in readonly mode results in ReadOnlyCell or EmptyCell objects being returned.

>>> workbook = openpyxl_excel_reader.load_workbook(file, read_only=True)
>>> worksheet = workbook.worksheets[0]  # openpyxl.worksheet._read_only.ReadOnlyWorksheet
>>> [row for row in worksheet.rows]
[
    (<ReadOnlyCell 'Sheet1'.A1>, <EmptyCell>, <ReadOnlyCell 'Sheet1'.C1>),
    (<ReadOnlyCell 'Sheet1'.A2>, <EmptyCell>, <ReadOnlyCell 'Sheet1'.C2>),
]

Locate the openpyxl stub defining worksheet.rows and inspect how the regular and read-only worksheet cell types are represented. Update the return typing to account for ReadOnlyCell and EmptyCell, then run the relevant type checks or tests to confirm both worksheet modes are covered.