fix stalled subprocess close

[Mike8]

This fix is possibly related to the problem described in #94 by @yanivy-nr.

It happens from time to time that subprocess close() stalls forever. I guess the reason is a raise conditions when terminating the process and closing the piped streams.

A note in Python's subprocess documentation suggests to use following code snipped to properly terminate/kill a subprocess:

proc = subprocess.Popen(...)
try:
    outs, errs = proc.communicate(timeout=15)
except TimeoutExpired:
    proc.kill()
    outs, errs = proc.communicate()

[codecov[bot]]

Codecov Report

Patch coverage is 40.00% of modified lines.

Files Changed	Coverage
tbot/machine/channel/subprocess.py	40.00%

:loudspeaker: Thoughts on this report? Let us know!.