ci(setup-python): specify explicit python-version

actions/setup-python@v4 no longer has a default; one must be specified
using `python-versio` or `python-version-file`.

https://github.com/actions/setup-python/releases/tag/v4.0.0

Author: scop

.github/workflows/ci.yaml

@@ -12,6 +12,8 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-python@v4
+        with:
+          python-version: ">=3.6"
       - uses: actions/cache@v3
         with:
           path: ~/.cache/pip