run-tests.yml: drop python 3.7 from matrix.

Currently, Python 3.7 in GitHub Actions (on macos-latest) appears to
be broken ("No module named '_bz2'").  Moreover, this version has now
reached its end of life.  Remove it from the test matrix.

Author: Benjamin Moody

.github/workflows/run-tests.yml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       matrix:
         os: [windows-latest, ubuntu-latest, macos-latest]
-        python-version: ["3.7", "3.8", "3.9", "3.10"]
+        python-version: ["3.8", "3.9", "3.10"]
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python ${{ matrix.python-version }}