return type of get_date() to -> datetime.date

[meni181818]

as described in the comment.
"""Return the content of the DateEntry as a datetime.date instance."""

[photodude]

Seems the failures in the 3.6 and 3.7 tests may be related to something specific to the tests when ran in those versions.

ERROR: test_calendar_textvariable (tests.test_calendar.TestCalendar)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\projects\tkcalendar\tests\test_calendar.py", line 228, in test_calendar_textvariable
    year=2015, month=1, day=3, textvariable=var)
  File "C:\projects\tkcalendar\tkcalendar\calendar_.py", line 268, in __init__
    self._sel_date = parse_date(self._textvariable.get(), locale)
  File "C:\PYTHON36\lib\site-packages\babel\dates.py", line 1182, in parse_date
    raise ParseError("No numbers were found in input")
babel.dates.ParseError: No numbers were found in input