Update python version to 3.7

See changes [here](https://github.com/pi-hole/pi-hole/pull/3625/files#diff-ef2cef9f88b4fe09ca3082140e67f5ad34fb65fb6e228f119d3812261ae51449R2)

Author: PromoFaux

build-configs/core.yml

@@ -5,7 +5,7 @@ services:
   - docker
 language: python
 python:
-  - "3.6"
+  - "3.7"
 install:
   - pip install -r requirements.txt
 script: