deps: add imageio-ffmpeg requirement to install.py

Fixes yownas#41 by adding imageio-ffmpeg to extension requirements/dependencies.

This is required due to some changes with moviepy's underlying imageio
library, which has deprecated/removed the old autoinstallation method
around 2019-ish, and as such moviepy is no longer capable of
autoinstalling ffmpeg.

See Zulko/moviepy#906 and Zulko/moviepy#908
for further details on the moviepy side.

This does result in downloading ffmpeg on systems where it may already
be installed, but it's a reasonably small download and on some systems
(eg Ubuntu 22.04) moviepy 1.0.3 fails to pick up system ffmpeg anyway.

install.py

@@ -2,3 +2,6 @@
 
 if not launch.is_installed("moviepy"):
     launch.run_pip("install moviepy==1.0.3", "requirements for Seed Travel")
+
+if not launch.is_installed("imageio-ffmpeg"):
+    launch.run_pip("install imageio-ffmpeg", "requirements for Seed Travel")