Fails on multi-valued dates and times

[Blair Conrad]

Example:

(0018, 1200) Date of Last Calibration            DA: ['19900101', '19900101']
(0018, 1201) Time of Last Calibration            TM: ['010000.000000', '010000.000000']

gives

Traceback (most recent call last):
  File "c:\program files\python37\lib\site-packages\pydicom\tag.py", line 30, in tag_in_exception
    yield
  File "c:\program files\python37\lib\site-packages\pydicom\dataset.py", line 1354, in walk
    callback(self, data_element)  # self = this Dataset
  File "c:\program files\python37\lib\site-packages\dicognito\anonymizer.py", line 114, in _anonymize_element
    if handler(dataset, data_element):
  File "c:\program files\python37\lib\site-packages\dicognito\datetimeanonymizer.py", line 42, in __call__
    self._anonymize_date_and_time(dataset, data_element)
  File "c:\program files\python37\lib\site-packages\dicognito\datetimeanonymizer.py", line 51, in _anonymize_date_and_time
    old_date = datetime.datetime.strptime(date_value, date_format).date()
TypeError: strptime() argument 1 must be str, not MultiValue