py/objdict: Quote non-string types when used as keys in JSON output.

JSON requires that keys of objects be strings.  CPython will therefore
automatically quote simple types (NoneType, bool, int, float) when they are
used directly as keys in JSON output.  To prevent subtle bugs and emit
compliant JSON, MicroPython should at least test for such keys so they
aren't silently let through.  Then doing the actual quoting is a similar
cost to raising an exception, so that's what is implemented by this patch.

Fixes issue #4790.