py-jsonschema: updated to 4.17.3

v4.17.3
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* Fix instantiating validators with cached refs to boolean schemas
  rather than objects.

v4.17.2
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* Empty strings are not valid relative JSON Pointers (aren't valid under the
  RJP format).
* Durations without (trailing) units are not valid durations (aren't
  valid under the duration format). This involves changing the dependency
  used for validating durations (from ``isoduration`` to ``isodate``).

(adam)