Remove obsolete reference to ensure_python

jupyter_packaging deprecates ensure_python and recommends using python_requres.
We already use python_requires, so there is no need to also use ensure_python.
Apparently, there are problems on Python 3.10.

Fixes jupyter-widgets#3364

jupyterlab_widgets/setup.py

@@ -5,7 +5,7 @@
 
 from jupyter_packaging import (
     create_cmdclass, install_npm, ensure_targets,
-    combine_commands, ensure_python, get_version,
+    combine_commands, get_version,
 )
 import setuptools
 
@@ -14,9 +14,6 @@
 # The name of the project
 name = "jupyterlab_widgets"
 
-# Ensure a valid python version
-ensure_python(">=3.6")
-
 # Get our version
 version = get_version(os.path.join(name, "_version.py"))