FIX: Special handling for ambiguous date formats

[TroyDowling]

This small patch revises the string_to_date function in typing.py to more gracefully handle parsing dates without a year. The motivation being to address the DeprecationWarning issued by Python in recent versions (and to avoid a possible breakage in the upcoming Python 3.15).

This closes #335 on the bug board.

• Interpolates the placeholder year 1900 for the two ambiguous formats.
• Formatted with ruff.

Before

................./home/troy/projects/khard/khard/helpers/typing.py:76: DeprecationWarning: Parsing dates i
nvolving a day of month without a year specified is ambiguous
and fails to parse leap day. The default behavior will change in Python 3.15
to either always raise an exception or to use a different default year (TBD).
To avoid trouble, add a specific year to the input & format.
See https://github.com/python/cpython/issues/70647.
  return datetime.strptime(string, fmt)
.................../home/troy/projects/khard/khard/cli.py:208: PendingDeprecationWarning: FileType is depr
ecated. Simply open files after parsing arguments.
  type=argparse.FileType("w"),
..........................................................................................................
....................................................................................ss....................
.....................................................................x....................................
.............................................
----------------------------------------------------------------------
Ran 399 tests in 0.326s

OK (skipped=2, expected failures=1)

After

..................................../home/troy/projects/khard/khard/cli.py:208: PendingDeprecationWarning:
 FileType is deprecated. Simply open files after parsing arguments.
  type=argparse.FileType("w"),
..........................................................................................................
....................................................................................ss....................
.....................................................................x....................................
.............................................
----------------------------------------------------------------------
Ran 399 tests in 0.327s

OK (skipped=2, expected failures=1)

and

$ python -W error::DeprecationWarning -m unittest test.test_helpers_typing.StringToDate
..........
----------------------------------------------------------------------
Ran 10 tests in 0.003s

OK