Add dependabot config to update python dependencies

Author: dstansby

.github/dependabot.yml

@@ -9,3 +9,12 @@ updates:
       actions:
         patterns:
           - "*"
+
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    groups:
+      python:
+        patterns:
+          - "*"