Skip tests for Python3.

@jerome 
@vpelletier ,

Tests do pass at
https://nexedi.erp5.net/test_result_module/20200122-713A91C9/view?ignore_layout:int=1

/reviewed-on !12

caucase/test.py

@@ -305,6 +305,7 @@ def captureStdout():
   finally:
     sys.stdout = orig_stdout
 
+@unittest.skipIf(sys.version_info >= (3, ), 'Caucase currently supports python 2 only')
 class CaucaseTest(unittest.TestCase):
   """
   Test a complete caucase setup: spawn a caucase-http server on CAUCASE_NETLOC