ci: test pypy 3.9 (#3789)

* ci: test pypy 3.9

* ci: try a use of FindPython with PyPy

.github/workflows/ci.yml

@@ -32,6 +32,7 @@ jobs:
         - '3.10'
         - 'pypy-3.7'
         - 'pypy-3.8'
+        - 'pypy-3.9'
 
         # Items in here will either be added to the build matrix (if not
         # present), or add new keys to an existing matrix element if all the
@@ -45,6 +46,10 @@ jobs:
             args: >
               -DPYBIND11_FINDPYTHON=ON
               -DCMAKE_CXX_FLAGS="-D_=1"
+          - runs-on: ubuntu-latest
+            python: 'pypy-3.9'
+            args: >
+              -DPYBIND11_FINDPYTHON=ON
           - runs-on: windows-2019
             python: '3.6'
             args: >