Require ldap3 2.9.1+ released 2021 (currently latest) as a lower bound

By introducing a lower bounded version for the ldap3 dependency, future
maintenance could become easier as its clear whats meant to be
supported. With this, it becomes possible to introduce tests against the
oldest supported dependencies etc.

setup.py

@@ -14,7 +14,7 @@
     python_requires=">=3.9",
     install_requires=[
         "jupyterhub>=4.1.6",
-        "ldap3",
+        "ldap3>=2.9.1",
         "traitlets",
     ],
     extras_require={