Update README for Python 3

[jfthuong]

README displays the code example print c.get_symbol('GBP'): it should be updated to Python 3 (python(...)).

The same remark applies to doc in https://forex-python.readthedocs.io/en/latest/usage.html.

>>> c.get_symbol('GBP')
u'\xa3'
>>> print c.get_symbol('GBP')
£
>>> print c.get_symbol('EUR')
€