[AIRFLOW-520] Fix Version Info in Flask UI (#4072)

Author: kaxil

airflow/www/views.py

@@ -2845,7 +2845,7 @@ class VersionView(wwwutils.SuperUserMixin, BaseView):
     def version(self):
         # Look at the version from setup.py
         try:
-            airflow_version = pkg_resources.require("apache-airflow")[0].version
+            airflow_version = airflow.__version__
         except Exception as e:
             airflow_version = None
             logging.error(e)