ci: switch unittests from cpython to pypy

work around cpython 3.10 incompatibility with bencoder.pyx

- whtsky/bencoder.pyx#143

Author: 3np

.github/workflows/unittests.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       matrix:
         os: [macos-13, ubuntu-latest]
-        python-version: ["3.8", "3.12"]
+        python-version: ["pypy3.8", "pypy3.12"]
         bitcoind-version: ["25.1", "29.0"]
 
     steps: