[update] update pytest.yml

Author: yyq2020

.github/workflows/pytest.yml

@@ -26,7 +26,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
         with:
-          python-version: '3.x' # Version range or exact version of a Python version to use, using SemVer's version range syntax
+          python-version: '3.9' # Version range or exact version of a Python version to use, using SemVer's version range syntax
           architecture: 'x64' # optional x64 or x86. Defaults to x64 if not specified
           cache: 'pip'