Pin CI Python 3.7 to 3.7.16 as workaround

The GitHub Actions CI for macOS with Python 3.7 leads to a `_bz2` missing module error (see actions/setup-python#682). Fix by pinning this Python to the prior version (eg see opendilab/DI-engine@f2881fe).

.github/workflows/main.yml

@@ -23,7 +23,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-20.04, ubuntu-latest, macos-latest, windows-latest]
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.6, 3.7.16, 3.8, 3.9]
         exclude:
           - os: ubuntu-20.04
             python-version: 3.7