cap numpy to <1.24.0

OK so now numpy is the culprit (shipped unbound via ocrd) which had several deprecations expire with release of v1.24.0 that require changes to our codebase, e.g. 
* The deprecation for the aliases np.object, np.bool, np.float, np.complex, np.str, and np.int is expired
* Ragged array creation will now always raise a ValueError unless dtype=object is passed.

See also here: https://numpy.org/devdocs/release/1.24.0-notes.html#expired-deprecations

requirements.txt

@@ -1,5 +1,6 @@
 # ocrd includes opencv, numpy, shapely, click
 ocrd >= 2.23.3
+numpy <1.24.0
 scikit-learn >= 0.23.2
 tensorflow >= 2.4.0, <2.12.0
 imutils >= 0.5.3