test_argcomplete: remove usage of distutils.spawn

[blueyed]

Fixes collection error with Python 3.5.3 (Travis):

testing/test_parseopt.py:2: in <module>
    import distutils.spawn
.tox/py35-coverage/lib/python3.5/distutils/__init__.py:4: in <module>
    import imp
.tox/py35-coverage/lib/python3.5/imp.py:33: in <module>
    PendingDeprecationWarning, stacklevel=2)
E   PendingDeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses

Build log: https://travis-ci.org/blueyed/pytest/builds/648305304

[bluetech]

Maybe better to just use shutil.which for this? https://docs.python.org/3/library/shutil.html#shutil.which

[blueyed]

Maybe better to just use shutil.which for this? https://docs.python.org/3/library/shutil.html#shutil.which

Sure, done.