Build and returns a URL used to make a Dark Sky API call.

def url(self):
        # type:() -> str
        """
        Build and returns a URL used to make a Dark Sky API call.
        """
        url = "https://api.darksky.net/forecast/{key}/{lat},{lon}".format(key=self.api_key,
                                                                          lat=self.latitude,
                                                                          lon=self.longitude)

        if isinstance(self._date, datetime):
            url += ",{:%Y-%m-%dT%H:%M:%S}".format(self._date)

        url += "?units={}".format(self.units)

        if self.lang != "auto":
            url += "&lang={}".format(self.lang)

        if len(self._exclude) > 0:
            url += "&exclude="
            for e in self._exclude:
                url += "{},".format(e)
            url = url.strip(",")

        if self._extend:
            url += "&extend=hourly"

        return url