[OE-core] iotop: failure, python module subprocess is missing

[Isaac Nickaein]

> Yes.  Should be a simple fix. :)

As a workaround, I added  iotop_%.bbappend to my layer with following content:

RDEPENDS_${PN} += " python-subprocess "

This fixed the issue but I'm not sure if this is the *correct* fix.
There could be some other bug or some other higher-level missing
dependency that could be overseen.