Add try/except handler for graceful import handler

Add try/except handler for handling imports from different SQLAlchemy project versions.

flask_admin/contrib/sqla/tools.py

 import types
 
 from sqlalchemy import tuple_, or_, and_, inspect
-from sqlalchemy.orm.clsregistry import _class_resolver
+try:
+    # Attempt _class_resolver import from SQLALchemy 1.4/2.0 module architecture.
+    from sqlalchemy.orm.clsregistry import _class_resolver
+except ImportError:
+    # If 1.4/2.0 module import fails, fall back to <1.3.x architecture.
+    from sqlalchemy.ext.declarative.clsregistry import _class_resolver
 from sqlalchemy.ext.hybrid import hybrid_property
 from sqlalchemy.ext.associationproxy import ASSOCIATION_PROXY
 from sqlalchemy.sql.operators import eq