crds issues warning on python 3.12 due to ast.Str deprecation

[braingram]

The following warning was seen in a python 3.12 test run in stdatamodels:
https://github.com/spacetelescope/stdatamodels/actions/runs/6611014333/job/18095432388

E               crds.core.exceptions.MappingError: Can't load file jwst_nirspec_apcorr_0009.rmap : ast.Str is deprecated and will be removed in Python 3.14; use ast.Constant instead

Possibly due to usage here:

crds/crds/core/mapping_verifier.py

Line 167 in d5d92a1

self.assert_(node, isinstance(node.value, (ast.Call, ast.Dict, ast.Str)),