Timezone-aware datetimes raise an Exception in version 3.0.7
Update for dates done in PR!399 introduced a change that raise an exception for aware dates like `datetime(2021, 1, 1).replace(tzinfo=timezone.utc).astimezone(tz=None)`: ```python ... File "C:\Python37-32\lib\site-packages\openpyxl\cell\_writer.py", line 76, in lxml_write_cell value, attributes = _set_attributes(cell, styled) File "C:\Python37-32\lib\site-packages\openpyxl\cell\_writer.py", line 32, in _set_attributes value = to_excel(value, cell.parent.parent.epoch) File "C:\Python37-32\lib\site-packages\openpyxl\utils\datetime.py", line 97, in to_excel days = (dt - epoch).days TypeError: can't subtract offset-naive and offset-aware datetimes ``` The following code can reproduce the issue: ```python from datetime import datetime, timezone, timedelta import calendar from openpyxl import Workbook def utc_to_local(utc_dt): """Transform date to Local Timezone""" # get integer timestamp to avoid precision lost timestamp = calendar.timegm(utc_dt.timetuple()) local_dt = datetime.fromtimestamp(timestamp) assert utc_dt.resolution >= timedelta(microseconds=1) return local_dt.replace(microsecond=utc_dt.microsecond) def utc_to_local_simpler(utc_dt: datetime): """Transform date to Local Timezone""" return utc_dt.replace(tzinfo=timezone.utc).astimezone(tz=None) wb = Workbook() ws = wb.active ws.column_dimensions["A"].width = 25 date = datetime(2021, 1, 1) ws["A1"] = date # OK ws["A2"] = utc_to_local_simpler(date) # OK ws["A3"] = utc_to_local(date) # Exception dest_filename = "check_writing_dates.xlsx" wb.save(filename=dest_filename) ``` Note: I have `cell.parent.parent.iso_dates` equals to False in my case, in condition of [_writer.py:27](https://foss.heptapod.net/openpyxl/openpyxl/-/blob/9131b415394123e78b61a7bb48291e6731f9748d/openpyxl/cell/_writer.py#L27).
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