Use pypy2.7 for tests, since setup-python no longer has 2.7

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

Author: andersk

.github/workflows/tests-and-lint.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: [2.7, 3.7, 3.8, 3.9, "3.10", 3.11]
+        python-version: ["pypy2.7", "3.7", "3.8", "3.9", "3.10", "3.11"]
 
     steps:
       - uses: actions/checkout@v2