.github: remove old PyPy version on macOS

GitHub is migrating macos-latest to macos-11, and it only has pypy-3.7:
https://github.com/actions/virtual-environments/blob/45df250473c17be4ad5439675d73415ae94d7803/images/macos/macos-11-Readme.md#pypy

There are known issues about this:
- actions/setup-python#244
  "macos-latest (11.6) is not longer working with pypy3"
- https://github.com/actions/setup-python/actions/workflows/test-pypy.yml
  tests are failing

.github/workflows/run_tests.yml

@@ -15,7 +15,13 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9, pypy2, pypy3]
+        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9, pypy-2.7, pypy-3.6, pypy-3.7]
+        exclude:
+          # macos-11 only has pypy-3.7
+          - os: macos-latest
+            python-version: pypy-2.7
+          - os: macos-latest
+            python-version: pypy-3.6
 
     steps:
     - uses: actions/checkout@v2