Drop old PYTHON_VERSION_DEFAULT from CI (#60)

Author: TheJulianJES

.github/workflows/ci.yml

@@ -11,7 +11,6 @@ jobs:
       CODE_FOLDER: zigpy_cli
       CACHE_VERSION: 3
       PRE_COMMIT_CACHE_PATH: ~/.cache/pre-commit
-      PYTHON_VERSION_DEFAULT: 3.9.15
       MINIMUM_COVERAGE_PERCENTAGE: 1
     secrets:
       CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}