Python 3 compatibility fix

[carlthome]

'#' * x causes a TypeError in Python 3 when x is interpreted as a float. Enforcing it to be an int.

Traceback (most recent call last):
  File "test.py", line 56, in <module>
    pm = pretty_midi.PrettyMIDI(midi_path)
  File "/home/carl/anaconda3/lib/python3.5/site-packages/pretty_midi/pretty_midi.py", line 75, in __init__
    self._load_metadata(midi_data)
  File "/home/carl/anaconda3/lib/python3.5/site-packages/pretty_midi/pretty_midi.py", line 164, in _load_metadata
    event.data[1], event.get_alternatives()),
  File "/home/carl/anaconda3/lib/python3.5/site-packages/pretty_midi/utilities.py", line 145, in mode_accidentals_to_key_number
    key = sharp_keys[num_accidentals % 7] + '#' * num_sharps
TypeError: can't multiply sequence by non-int of type 'float'

[craffel]

Oops, right on, thanks. We need to add compatibility testing for Python 3.