AudioLeadIn not optional

[de-odex]

https://osu.ppy.sh/b/227 :P

Traceback (most recent call last):
  File "C:\Users\deodex\AppData\Local\Programs\Python\Python36\lib\site-packages\slider\library.py", line 243, in lookup_by_id
    return self._read_beatmap(self, beatmap_id=beatmap_id)
  File "C:\Users\deodex\AppData\Local\Programs\Python\Python36\lib\site-packages\slider\library.py", line 216, in _raw_read_beatmap
    return Beatmap.from_path(self._cache[f'id:{beatmap_id}'])
  File "C:\Users\deodex\AppData\Local\Programs\Python\Python36\lib\site-packages\slider\library.py", line 32, in __getitem__
    return d[key]
  File "C:\Users\deodex\AppData\Local\Programs\Python\Python36\lib\dbm\dumb.py", line 148, in __getitem__
    pos, siz = self._index[key]     # may raise KeyError
KeyError: b'id:227'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\deodex\AppData\Local\Programs\Python\Python36\lib\site-packages\slider\beatmap.py", line 767, in _get_as_str
    return mapping[field]
KeyError: 'AudioLeadIn'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "bot.py", line 173, in sendpp
    beatmap_data = osu_library.lookup_by_id(beatmap_id, download=True, save=True)
  File "C:\Users\deodex\AppData\Local\Programs\Python\Python36\lib\site-packages\slider\library.py", line 247, in lookup_by_id
    return self.download(beatmap_id, save=save)
  File "C:\Users\deodex\AppData\Local\Programs\Python\Python36\lib\site-packages\slider\library.py", line 360, in download
    beatmap = Beatmap.parse(data.decode('utf-8-sig'))
  File "C:\Users\deodex\AppData\Local\Programs\Python\Python36\lib\site-packages\slider\beatmap.py", line 1608, in parse
    milliseconds=_get_as_int(groups, 'General', 'AudioLeadIn'),
  File "C:\Users\deodex\AppData\Local\Programs\Python\Python36\lib\site-packages\slider\beatmap.py", line 794, in _get_as_int
    v = _get_as_str(groups, section, field, default)
  File "C:\Users\deodex\AppData\Local\Programs\Python\Python36\lib\site-packages\slider\beatmap.py", line 770, in _get_as_str
    raise ValueError(f'missing field {field!r} in section {section!r}')
ValueError: missing field 'AudioLeadIn' in section 'General'

[llllllllll]

fixed in 1f090da. thanks for the report!

I think we will uncover more issues like this in older beat maps but I am happy to fix them.