[refactoring] Migrate from Popen() to run()

[mips171]

From the Python docs

The recommended approach to invoking subprocesses is to use the run() function for all use cases it can handle. For more advanced use cases, the underlying Popen interface can be used directly.

The run() function was added in Python 3.5; if you need to retain compatibility with older versions, see the Older high-level API section.

Is this an "advanced use case"? Should we consider migrating to run()?

openwisp-monitoring/openwisp_monitoring/check/classes/ping.py

Line 140 in 1b78b37

p = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE)

[nemesifier]

@nickberry17 I'm not such in a hurry to change it, but should be just a 2 line change:

openwisp-monitoring/openwisp_monitoring/check/classes/ping.py

Lines 140 to 141 in 1b78b37

	p = subprocess.Popen(command, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
	return p.communicate()

Initially this module supported also python2 but now we should be able to simplify that.

[Aryamanz29]

I would like to work on this @nemesisdesign