ujson?

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• do we only use json features that are available in ujson?
• should we make ujson a proper dependency?
• if we keep it optional, should we use it everywhere (e.g., the handlers) if it's installed?*
• or should we just drop the try/except import in mflow.py since it's not worth it?

---

*it would be easy to add a shim xjson.py (or similar):

try:
    from ujson import *
except ImportError:
    from json import *

and do:

from . import xjson as json

or just add it to utils.py:

try:
    import ujson as json
except:
    import json

and do:

from .utils import json