Create missing pypyX.Y symlinks (#347)

`pypyX.Y.exe` executables are missing from PyPy archives on Windows before v7.3.9 (X.Y < 3.9)
`pypy2.7` symlinks are also missing from macOS/Linux PyPy archives before v7.3.9

relates to #346

.github/workflows/test-pypy.yml

@@ -44,3 +44,17 @@ jobs:
   
       - name: Run simple code
         run: python -c 'import math; print(math.factorial(5))'
+
+      - name: Assert PyPy is running
+        run: |
+          import platform
+          assert platform.python_implementation().lower() == "pypy"
+        shell: python
+
+      - name: Assert expected binaries (or symlinks) are present
+        run: |
+          EXECUTABLE=${{ matrix.pypy }}
+          EXECUTABLE=${EXECUTABLE/-/}  # remove the first '-' in "pypy-X.Y" -> "pypyX.Y" to match executable name
+          EXECUTABLE=${EXECUTABLE%%-*}  # remove any -* suffixe
+          ${EXECUTABLE} --version
+        shell: bash