bpo-35066: _dateime.datetime.strftime copies trailing '%' (GH-10692)

bpo-35066: _dateime.datetime.strftime copies trailing '%' (GH-10692)

Previously, calling the strftime() method on a datetime object with a
trailing '%' in the format string would result in an exception. However,
this only occured when the datetime C module was being used; the python
implementation did not match this behavior. Datetime is now PEP-399
compliant, and will not throw an exception on a trailing '%'.
(cherry picked from commit 454b3d4ea246e8751534e105548d141ed7b0b032)

Co-authored-by: MichaelSaah <mike.saah@gmail.com>