[Windows][melodic] Python 3 compatibility.

[seanyen]

On Python3 & Windows, the code path of socketserver.ForkingMixIn will be exercised and reported as an exception (shown below) because it is not available on Windows.

(py37) c:\workspace\conda_ros>roscore
Traceback (most recent call last):
  File "C:\workspace\conda_ros\install_isolated\bin\roscore", line 36, in <module>
    from rosmaster.master_api import NUM_WORKERS
  File "C:\workspace\conda_ros\install_isolated\lib\site-packages\rosmaster\__init__.py", line 35, in <module>
    from .main import rosmaster_main
  File "C:\workspace\conda_ros\install_isolated\lib\site-packages\rosmaster\main.py", line 43, in <module>
    import rosmaster.master
  File "C:\workspace\conda_ros\install_isolated\lib\site-packages\rosmaster\master.py", line 45, in <module>
    import rosgraph.xmlrpc
  File "C:\workspace\conda_ros\install_isolated\lib\site-packages\rosgraph\xmlrpc.py", line 133, in <module>
    class ForkingXMLRPCServer(socketserver.ForkingMixIn, SimpleXMLRPCServer):
AttributeError: module 'socketserver' has no attribute 'ForkingMixIn'

In this pull request, I am proposing to use conditional check for it.

[seanyen]

cc @dirk-thomas

[seanyen]

The test failure seems not to do with this change:

float division by zero
  File "/usr/lib/python2.7/unittest/case.py", line 329, in run
    testMethod()
  File "/tmp/ws/src/ros_comm/test/test_rospy/test/rostest/test_topic_statistics.py", line 91, in test_frequencies
    self.assertTrue(self.frequency_acceptable('/very_slow_chatter', 0.5))
  File "/tmp/ws/src/ros_comm/test/test_rospy/test/rostest/test_topic_statistics.py", line 70, in frequency_acceptable
    found_freq = 1.0 / msg.period_mean.to_sec()

[seanyen]

@mikepurvis Happy new year! Hope I am pinging the correct owner. This one is ready for review and merge.

[mikepurvis]

I see no instances of ForkingXMLRPCServer actually being used in our codebase; I don't know the history of why it's here. Generating it only optionally seems perfectly fine.

[mikepurvis]

Great, thanks.