CI: Add PyPy 3.7 and 3.10 to test matrix

Author: amotl

.github/workflows/tests.yml

@@ -29,6 +29,13 @@ jobs:
         ]
         python-version: ["3.7", "3.11", "3.12"]
 
+        # Run PyPy tests on Linux only.
+        include:
+          - os: 'ubuntu-22.04'
+            python-version: 'pypy-3.7'
+          - os: 'ubuntu-22.04'
+            python-version: 'pypy-3.10'
+
         # Doesn't work on macOS (ARM) with older versions of Python.
         exclude:
           - os: 'macos-latest'