Minor fix to Python binding for ParamValueList (#2638)
Minor fix to Python binding for ParamValueList (#2638)

The `__getitem__` method taking an integer index correctly set the
return value policy so it returns in internal reference rather than
a copy, but the `__getitem__` method taking a string did not.

Also fix indentation.

Signed-off-by: Larry Gritz <f0a1ce6f80112d72f7276457fd261d671b827567@larrygritz.com>
lgritz/oiio,lgritz/oiio,OpenImageIO/oiio,lgritz/oiio,lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio,OpenImageIO/oiio