Support for Python 3

[sh-mykola]

Remove unicode variable in pyairports.py:

How it must to be:

line 48:
@staticmethod
    def _validate(iata):
        if not isinstance(iata, str):
            raise ValueError("iata must be a string, it is a {0}".format(type(iata)))
...

How it is:

line 48:
@staticmethod
    def _validate(iata):
        if not isinstance(iata, (str, unicode)):
            raise ValueError("iata must be a string, it is a {0}".format(type(iata)))
...