build: avoid python 2.4 build failure

On RHEL 5, I got:

/usr/bin/python ./generator.py /usr/bin/python
  File "./generator.py", line 427
    "virStreamFree", # Needed in custom virStream __del__, but free shouldn't
                   ^
SyntaxError: invalid syntax

* python/generator.py (function_skip_python_impl): Use same syntax
as other skip lists.

generator.py

@@ -423,10 +423,10 @@ skip_function = (
 )
 
 # Generate C code, but skip python impl
-function_skip_python_impl = {
+function_skip_python_impl = (
     "virStreamFree", # Needed in custom virStream __del__, but free shouldn't
                      # be exposed in bindings
-}
+)
 
 function_skip_index_one = (
     "virDomainRevertToSnapshot",