Fix Python 3.13 enum incompatibility in string format

[Thyre]

Using a value of None for the FunctionFormat NONE causes Python 3.13 to abort Extra-P with the following error message:

[...]
    from extrap.util.string_formats import FunctionFormats
  File "extrap/util/string_formats.py", line 11, in <module>
    class FunctionFormats(enum.Enum):
    ...<2 lines>...
        LATEX = enum.auto()
  File "extrap/util/string_formats.py", line 13, in FunctionFormats
    PYTHON = enum.auto()
    ^^^^^^
  File "python3.13/enum.py", line 441, in __setitem__
    v.value = self._generate_next_value(
              ~~~~~~~~~~~~~~~~~~~~~~~~~^
            key, 1, len(self._member_names), self._last_values[:],
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
            )
            ^
  File "python3.13/enum.py", line 1267, in _generate_next_value_
    raise TypeError('unable to increment %r' % (last_value, )) from None
TypeError: unable to increment (None,)

To fix this, also treat None as enum.auto(), so that values can be properly initialized.