reverse py_to_json integer types to match json::parse()

[maloel]

During debugging I noticed weird behavior. My conclusion:

• nlohmann::json and pybind11_json have different interpretation of number values
  • I.e., a 0 converted from Python is taken as a signed integer, while a text "0" is parsed as unsigned

So I made the python-to-json code match this behavior:

• An integer number (in Python, these are unbound) is converted to an 'unsigned` json value if it's not negative