fixed RGB modes silently depending on RGB images

Thanks to @Chris_J_Baird on Twitter reporting the issue:

> ColorSSTV.encode_line assuming its gets a tuple from __getpixel__
> had to make the Image.open() do a convert('RGB')

https://twitter.com/i/web/status/1149331458189737988

pysstv/color.py

@@ -11,7 +11,7 @@ RED, GREEN, BLUE = range(3)
 
 class ColorSSTV(GrayscaleSSTV):
     def on_init(self):
-        self.pixels = self.image.load()
+        self.pixels = self.image.convert('RGB').load()
 
     def encode_line(self, line):
         msec_pixel = self.SCAN / self.WIDTH