avoid TB on python3 due to strict dict type

On python3, we need to perform a union (not addition) on
dictionary sets to avoid...

Traceback (most recent call last):
  File "/home/sandbox/test/functional/fetch_tests.py", line 95, in test_with_releaser
    self.source_filename)
  File "/home/sandbox/test/functional/fixture.py", line 103, in tito
    return CLI().main(argstring.split(' '))
  File "/home/sandbox/src/tito/cli.py", line 222, in main
    return module.main(argv)
  File "/home/sandbox/src/tito/cli.py", line 599, in main
    **kwargs)
  File "/home/sandbox/src/tito/release/main.py", line 359, in __init__
    prefix="yumrepo-", **kwargs)
  File "/home/sandbox/src/tito/release/main.py", line 238, in __init__
    auto_accept, **kwargs)
  File "/home/sandbox/src/tito/release/main.py", line 73, in __init__
    _args = config_builder_args.items() + kwargs['builder_args'].items()
nose.proxy.TypeError: TypeError: unsupported operand type(s) for +: 'dict_items' and 'dict_items'

src/tito/compat.py

@@ -48,6 +48,18 @@ def getoutput(cmd):
     return getstatusoutput(cmd)[1]
 
 
+def dictionary_override(d1, d2):
+    """
+    Return a new dictionary object where
+    d2 elements override d1 elements.
+    """
+    if PY2:
+        overrides = d1.items() + d2.items()
+    else:
+        overrides = d1.items() | d2.items()
+    return dict(overrides)
+
+
 def write(fd, str):
     """
     A version of os.write that

src/tito/release/main.py

@@ -71,8 +71,11 @@ class Releaser(ConfigObject):
 
         # Override with builder args from command line if any were given:
         if 'builder_args' in kwargs:
-            self.builder_args = dict(config_builder_args.items() +
-                kwargs['builder_args'].items())
+            # (in case of dupes, last one wins)
+            self.builder_args = dictionary_override(
+                config_builder_args,
+                kwargs['builder_args']
+            )
         else:
             self.builder_args = config_builder_args