test_argcomplete: remove usage of distutils.spawn (#6703)

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

Author: blueyed

testing/test_parseopt.py

@@ -1,7 +1,7 @@
 import argparse
-import distutils.spawn
 import os
 import shlex
+import shutil
 import sys
 
 import py
@@ -291,7 +291,7 @@ def test_multiple_metavar_help(self, parser):
 
 
 def test_argcomplete(testdir, monkeypatch):
-    if not distutils.spawn.find_executable("bash"):
+    if not shutil.which("bash"):
         pytest.skip("bash not available")
     script = str(testdir.tmpdir.join("test_argcomplete"))