_parse_sentence may fail with an UnicodeError after a soft reboot

[MSube]

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code.py output:
Traceback (most recent call last):
File "code.py", line 7, in
File "adafruit_gps.py", line 102, in update
File "adafruit_gps.py", line 148, in _parse_sentence
UnicodeError:

line 148: sentence = str(sentence, 'ascii').strip()
fails if sentence contains stale data that is not ascii.

[evaherrada]

Can you check and see if this still happens? #21 should have fixed this