chore)tests): do not install rapidjson on PyPy

(Because it does not seem to build on PyPy cleanly with setuptools>=58 any more.)

requirements/tests

@@ -22,7 +22,8 @@ mujson
 ujson
 
 # it's slow to compile on emulated architectures; wheels missing for some EoL interpreters
-python-rapidjson; platform_machine != 's390x' and platform_machine != 'aarch64'
+# (and there is a new issue with building on PyPy in Actions, but we don't really need to test it with PyPy)
+python-rapidjson; platform_python_implementation != 'PyPy' and platform_machine != 's390x' and platform_machine != 'aarch64'
 
 # wheels are missing some EoL interpreters and non-x86 platforms; build would fail unless rust is available
 orjson; platform_python_implementation != 'PyPy' and platform_machine != 's390x' and platform_machine != 'aarch64'