minor fixes

[marc1uk]

• stray 'f' in print statement was throwing a syntax error
• '{self.mac_address}' was just being printed as a literal string, rather than the actual address.
• mode 'RGB' seemed to give varying results with transparent pngs - some would show transparent areas as black, others as white. Not sure why or what the difference was between the files - my image viewer showed both as transparent PNGs? Replacing with RGBA seemed to do the trick, though.

[HoroTW]

Thank you for the fix - on the f string part: could you elaborate the issue? Do you use an old python version (pre 3.6 [december 2016])? I can not See why the f string would be a syntax error.

[marc1uk]

Hi HoroTW, you're right, i'm running an old python version (3.5.3) which falls over with this printf syntax. i'm not a python user, so didn't realise that was valid with newer python. Feel free to drop that from the commit... although perhaps if it works in newer pythons too, it's better for compatibility.

[HoroTW]

Merged it 🙂 - I reverted the print back to use the f-strings, I'm with you on the backwards compability but I wan't to be able to use other features from 3.6 and really like the f-string style.