Description of problem:
A customer needed to extend the LDAP schema.
When trying to add a matching rule, the command fails.
This works fine when the matching rule option is not used.

Version-Release number of selected component (if applicable):
$ cat /etc/redhat-release 
Red Hat Enterprise Linux release 8.7 (Ootpa)
$

$ rpm -qa | grep 389-ds
389-ds-base-1.4.3.31-11.module+el8dsrv+17815+4f95348d.x86_64
cockpit-389-ds-1.4.3.31-11.module+el8dsrv+17815+4f95348d.noarch
389-ds-base-legacy-tools-1.4.3.31-11.module+el8dsrv+17815+4f95348d.x86_64
389-ds-base-libs-1.4.3.31-11.module+el8dsrv+17815+4f95348d.x86_64
$

How reproducible:
Always.

Steps to Reproduce:
For instance:

$ dsconf -j ldapi://%2fvar%2frun%2fslapd-<INSTANCE>.socket schema attributetypes add newAttr --syntax 1.3.6.1.4.1.1466.115.121.1.15 --equality caseexactmatch --single-value --user-mod --usage userApplications --desc Case_XXX
{
    "desc": "value has to be of str, was 'caseexactmatch'"
}
$

Customer was initially trying to add the "numericstringsubstringsmatch" to an existing attribute using the RHDS Console.
I could reproduce the issue with any type of matching rule.

# dsconf --verbose ldapi://%2fvar%2frun%2fslapd-<INSTANCE>.socket schema attributetypes add newAttr --syntax 1.3.6.1.4.1.1466.115.121.1.36 --substr numericstringsubstringsmatch --single-value --user-mod --usage userApplications --desc Test
...
vendorVersion: 389-Directory/1.4.3.31 B2023.012.1725

]
DEBUG: cn=schema get_attr_vals('attributeTypes')
DEBUG: value has to be of str, was 'numericstringsubstringsmatch'
Traceback (most recent call last):
  File "/usr/sbin/dsconf", line 138, in <module>
    result = args.func(inst, None, log, args)
  File "/usr/lib/python3.6/site-packages/lib389/cli_conf/schema.py", line 137, in add_attributetype
    schema.add_attributetype(parameters)
  File "/usr/lib/python3.6/site-packages/lib389/schema.py", line 344, in add_attributetype
    return self._add_schema_object(parameters, AttributeType)
  File "/usr/lib/python3.6/site-packages/lib389/schema.py", line 225, in _add_schema_object
    return self.add(attr_name, str(schema_object))
  File "/usr/lib64/python3.6/site-packages/ldap/schema/models.py", line 305, in __str__
    result.append(self.key_attr('SUBSTR',self.substr))
  File "/usr/lib64/python3.6/site-packages/ldap/schema/models.py", line 69, in key_attr
    assert value is None or type(value)==str,TypeError("value has to be of str, was %r" % value)
AssertionError: value has to be of str, was 'numericstringsubstringsmatch'
ERROR: Error: value has to be of str, was 'numericstringsubstringsmatch'
$

Actual results:
Failing to modify the custom schema.

Expected results:
Allow to add matching rules.

Additional info: