{command, feeders, diff}: replaces subprocess.Popen by .run

Popen requires thread/IO micromanaging, and locks some interactions
with multiprocess lib due to its combined behavior with subprocess.PIPE, being
unusable with .map function. Subprocess.run simplifies command execution
by managing threads/IO and is also recommended over Popen since Python
3.5.

As a result, stderr and stdout were also simplified.

Signed-off-by: Juliana Oliveira <juliana.orod@gmail.com>
Signed-off-by: Mattia Rizzolo <mattia@debian.org>