feat(ci): Added pipeline for tests

Add all supported, meaning by flaschengeist, python versions.
Use slim image of python instead of alpine, because Pillow does not
provide any wheels for musllibc

.woodpecker/lint.yml

@@ -1,6 +1,6 @@
 pipeline:
   lint:
-    image: python:alpine
+    image: python:slim
     commands:
       - pip install black
       - black --check --line-length 120 --target-version=py37 .

.woodpecker/test.yml

@@ -0,0 +1,18 @@
+pipeline:
+  install:
+    image: python:${PYTHON}-slim
+    commands:
+      - python -m pip install --upgrade pip
+      - pip install -v ".[tests]"
+  test:
+    image: python:${PYTHON}-slim
+    commands:
+      - pytest
+
+
+matrix:
+  PYTHON:
+    - 3.10
+    - 3.9
+    - 3.8
+    - 3.7