🐛 Require NumPy < 1.24 due to np.str deprecation/error

NumPy has deprecated np.str (etc.) since NumPy 1.20, and since Numpy
1.24 throws an error. We can't currently update to Calamari 2 (and fix
the problem there if necessary), so stick to NumPy 1.23.x for now.

#87

requirements.txt

@@ -1,4 +1,5 @@
 tensorflow >= 2.5.0
+numpy =~ 1.23.5  # XXX https://github.com/OCR-D/ocrd_calamari/issues/87
 calamari-ocr == 1.0.*
 setuptools >= 41.0.0  # tensorboard depends on this, but why do we get an error at runtime?
 click