Use .splitlines() instead of .split() for NoaaIntl

[DaStru]

When using the AirSigManager, I got the following error:

  File "c:\env\Lib\site-packages\avwx\current\airsigmet.py", line 247, in _bulletin
    type=Code(repr=type_key, value=BULLETIN_TYPES[type_key]),
                                   ~~~~~~~~~~~~~~^^^^^^^^^^
KeyError: 'VA'

After further investigation, I found out that the cause lies in the processing of the Data API message from aviationweather.gov for international SIGMETs.

These include the type of hazard in the first line, which is also taken into account in NoaaIntl. However, this first line is filtered out incorrectly.

This can be fixed by using .splitlines() instead of .split(), as it is probably also intended.