add python-json5 to support comments in json python parser

[prudhomm]

python json5 package allows comments in json files
this can be used as a replacement of json

[thomas-saigre]

@prudhomm I think this issue has been fixed. Moreover, I modified the function __init__.py.in (see feelpp/feelpp@1573e7e) so we don't need to use json5 anymore.