pyhon subprocess inherits stdin by default and causes ffmpeg to fail

[dale-wahl]

When running videohash as part of a program that has also used subprocess it seems to inherit the stdin and that can result in various failures for ffmpeg.

I have been documenting it here:
digitalmethodsinitiative/4cat#303 (comment)
Essentially, I can use videohash alone, but not with additional subprocesses unless I edit it and provide it with stdin=subprocess.DEVNULL since the default stdin is in use.

Sending a PR shortly with needed edit.