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Similarly to StrictUnicode mode (see b28613c2) add new opt-in mode that
requests to decode Python dicts as ogórek.Dict instead of builtin map.
As explained in recent patch "Add custom Dict that mirrors Python dict
behaviour" this is needed to fix decoding issues that can be there due
to different behaviour of Python dict and builtin Go map:

    ---- 8< ----
    Ogórek currently represents unpickled dict via map[any]any, which is
    logical, but also exhibits issues because builtin Go map behaviour is
    different from Python's dict behaviour. For example:

    - Python's dict allows tuples to be used in keys, while Go map
      does not (https://github.com/kisielk/og-rek/issues/50),

    - Python's dict allows both long and int to be used interchangeable as
      keys, while Go map does not handle *big.Int as key with the same
      semantic (https://github.com/kisielk/og-rek/issues/55)

    - Python's dict allows to use numbers interchangeable in keys - all int
      and float, but o...