renovate: Configure requirement file locations

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

renovate.json

@@ -1,5 +1,11 @@
 {
   "extends": [
     "config:base"
-  ]
+  ],
+  "pip_requirements": {
+    "fileMatch": [
+      "requirements(-\\w+)?\\.txt"
+    ]
+  },
+  "rangeStrategy": "widen"
 }