Python 2.7 compatibility fix

[puetro]

i am using this module on Libreelec on RPI3. The lightweight system behind KODI has only python 2.7. I sneak for a while and found solution to run this library on this older python. problem is int vs float:

line 176
self._sleep((highpulses * self.tx_pulselength) / 1000000)

update:
self._sleep((float(highpulses) * self.tx_pulselength) / 1000000)

the same fix on line 178.

problem: python 2.7 result of this calculations is always 0. the results should be for example 0.00035. without float is unreal.

worker for me to send commands.