drone matrix

.drone.yml

@@ -1,7 +1,7 @@
 ---
 pipeline:
   build:
-    image: python:3.5.2
+    image: python:${PY_VERSION}
     commands:
       - mkdir -p /tmp/
       - touch /tmp/try   # apparently needed
@@ -9,3 +9,10 @@ pipeline:
       - pip3 install -U pip
       - pip3 install .
       - python3 setup.py install test
+
+matrix:
+  PY_VERSION:
+    - 3.5
+    - 3.6
+    - 3.7
+    # slim and alpine variants do not work: error with mime types