Fix handling of function from standard library

[gpetretto]

Summary

I was trying to use a function from the standard library to create a simple example and I got an error:

import time
from jobflow import Job, SETTINGS

j = Job(time.sleep, (1,))
j.run(SETTINGS.JOB_STORE)

gives

TypeError: time.sleep() takes exactly one argument (2 given)

The reason is that sleep has a __self__ attribute and in Job it is then converted to a bound method. This seems to be shared by most (all?) the functions in the standard library.

To adress this I changed the check performed in Job.run() assuming that if the __self__ is a module, than the callable should not be converted to a method. I am not entirely sure if I may be missing some case where this is not true.

[utf]

Thanks @gpetretto