Fix openpyxl's workbook.pyi

[ghost]

Preperty active returns openpyxl.worksheet.worksheet.Worksheet or None. This can be seen from the source code:

    @property
    def active(self):
        """Get the currently active sheet or None

        :type: :class:`openpyxl.worksheet.worksheet.Worksheet`
        """
        try:
            return self._sheets[self._active_sheet_index]
        except IndexError:
            pass

_WorkbookChild does not have many of the attributes that Worksheet has.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[srittau]

Thank you for your contribution. Unfortunately, according to the implementation (and unlike what the docstring says), Workbook.active can be any kind of workbook child: Worksheet, WriteOnlyWorksheet, or Chartsheet. See also #9836.