enable github's dependabot for dev env

https://github.com/fphammerle/ical2vdir/commit/76bc605ceba2ba3db9001a43a752da1ff9b9491f
https://github.com/fphammerle/ical2vdir/commit/8ec47f9dcbd9a10da5a7f8ab8fdaddf855d4dec6
https://github.com/fphammerle/python-manchester-code/commit/88d7d6636f906293ac1a22fd31f42a144d28c6aa

.github/dependabot.yml

@@ -0,0 +1,14 @@
+# sync with https://github.com/fphammerle/ical2vdir/blob/master/.github/dependabot.yml
+
+version: 2
+
+updates:
+- # > Dependabot also supports the following package managers: [...]
+  # > - pipenv, pip-compile, and poetry (specify pip)
+  package-ecosystem: pip
+  directory: /
+  schedule:
+    interval: weekly
+    day: friday
+
+# https://docs.github.com/en/free-pro-team@latest/github/administering-a-repository/configuration-options-for-dependency-updates