Python 3 port: provide context to SimpleQueue when required

The multiprocessing library in Python 3 has some differences from the
one in Python 2.  One of those, is the requirement of a context for a
SimpleQueue.

Since we're going to use that specific interface (get_context()),
let's check for it, and use it to get a context if it exists.

PS: This approach for checking the interface avoids having an external
library like `six` imported here just for the reason of checking the
Python version.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/core/runner.py

@@ -570,7 +570,13 @@ class TestRunner(object):
         summary = set()
         if self.job.sysinfo is not None:
             self.job.sysinfo.start_job_hook()
-        queue = queues.SimpleQueue()
+
+        # Python 3 requires a context for a queue
+        if hasattr(multiprocessing, 'get_context'):
+            ctx = multiprocessing.get_context('spawn')
+            queue = queues.SimpleQueue(ctx=ctx)     # pylint: disable=E1123
+        else:
+            queue = queues.SimpleQueue()
 
         if timeout > 0:
             deadline = time.time() + timeout