Replace Bitcoin symbol with unicode

[kunambi]

A bug has been introduced in 1.6

Experiencing an error when using c.get_symbol("EUR")

  File "/usr/local/lib/python3.8/site-packages/forex_python/converter.py", line 137, in get_symbol
    currency_dict = self._get_data(currency_code)
  File "/usr/local/lib/python3.8/site-packages/forex_python/converter.py", line 125, in _get_data
    currency_data = json.loads(f.read())
  File "/usr/local/lib/python3.8/encodings/ascii.py", line 26, in decode
    return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 1252: ordinal not in range(128)

Created a pull request, !94