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).
issue