Pin setuptools for PyPy builds

Latest versions are having issues building the correct library on PyPy

Author: HexDecimal

pyproject.toml

@@ -3,6 +3,7 @@ requires = [
     # setuptools >=64.0.0 might break editable installs
     # https://github.com/pypa/setuptools/issues/3548
     "setuptools >=77.0.3",
+    'setuptools==84.1.0; implementation_name == "pypy"',
     "setuptools_scm[toml]>=6.2",
     "packaging>=24.2",
     "wheel>=0.37.1",