Remove the outdated Python 3.7 target in GH actions

Python 3.7 reached its end of life in June 2023 and is no longer
supported on the latest macOS target.

.github/workflows/main.yml

@@ -6,7 +6,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [3.7, 3.8, 3.9, '3.10', '3.11.5', '3.12', 'pypy-3.9']  # FIXME: forcing to use py3.11.5 since py3.11.7 causes an inexplicable failure in multiprocessing.
+        python-version: [3.8, 3.9, '3.10', '3.11.5', '3.12', 'pypy-3.9']  # FIXME: forcing to use py3.11.5 since py3.11.7 causes an inexplicable failure in multiprocessing.
     runs-on: ${{ matrix.os }}
     steps:
       - run: git config --global core.autocrlf input