Add deprecation warning for Python 3.5

docs/release-notes/version-2.7.md

@@ -1,5 +1,9 @@
 # v2.7.0 (FUTURE)
 
+!!! warning
+    NetBox v2.7 is the last major release that will support Python 3.5. Beginning with NetBox v2.8, Python 3.6 or higher
+    will be required.
+
 ## New Features
 
 ### Enhanced Device Type Import ([#451](https://github.com/netbox-community/netbox/issues/451))

netbox/netbox/settings.py

@@ -20,11 +20,17 @@ HOSTNAME = platform.node()
 # Set the base directory two levels up
 BASE_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 
-# Django 2.1+ requires Python 3.5+
+# Validate Python version
 if platform.python_version_tuple() < ('3', '5'):
     raise RuntimeError(
         "NetBox requires Python 3.5 or higher (current: Python {})".format(platform.python_version())
     )
+elif platform.python_version_tuple() < ('3', '6'):
+    warnings.warn(
+        "Python 3.6 or higher will be required starting with NetBox v2.8 (current: Python {})".format(
+            platform.python_version()
+        )
+    )
 
 
 #