Switch to renovate (#1044)

See marshmallow-code/marshmallow#2947

Author: sloria

.github/dependabot.yml

@@ -0,0 +1,4 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": ["github>marshmallow-code/renovate-config"]
+}