I donβt think that there is a slice notation exactly as described in your example, mainly because it openpyxluses lists Cell, not lists of simple values.
Here is the main thread that I would use to capture only part of the sheet.
>>> wb = openpyxl.load_workbook(file_name)
>>> sheet = wb.worksheets[0]
>>> rows = sheet.rows[1:3]
>>> foobar = [cell.value for row in rows for cell in row[3:10]]
EDIT: From this question, it looks like you can do something like this:
foo = [row[start_col:end_col] for row in sheet.rows[start_row:end_row]]
. foo[i][j].value Cell.