model.datatypes: fix error when copy Dates

[JGrothoff]

Currently, the copy of a submodel with Date properties using pythons built-in copy.copy or copy.deepcopy fails with the following error:

File "basyx-python-sdk\basyx\aas\model\datatypes.py", line 49, in new
res: "Date" = datetime.date.new(cls, year, month, day) # type: ignore # pickle support is not in typeshed
TypeError: 'bytes' object cannot be interpreted as an integer

The error can be reproduced with (see test added in this PR):

import copy
from basyx.aas import model

date = model.datatypes.Date(2020, 1, 24)
date_copy_shallow = copy.copy(date) # will Fail
date_copy_deep = copy.deepcopy(date) # will Fail

I don't know where the error comes from, but when the class is copied the year argument gets a bytestring and month as well as dayare None.
However, overwriting __copy__ for shallow copies and __deepcopy__ seems to solve the issue.

[s-heppner]

LGTM, thank you for your contribution!