use custom installer to properly resolve dependencies for PyPy

installer/cppyy_monkey_patch.py

@@ -0,0 +1,40 @@
+# monkey patch to be able to select a specific backend based on PyPy's version,
+# which is not possible in the pyproject.toml file as there is currently no
+# marker for it (this may change, after which this file can be removed)
+
+from setuptools.build_meta import *
+try:
+  # _BACKEND is the primary, __legacy__ the backwards compatible backend
+    from setuptools.build_meta import _BACKEND
+    main = _BACKEND
+except (NameError, ImportError):
+  # fallback as the name __legacy__ is actually documented (and part of __all__)
+    main = __legacy__
+
+# the following ensures proper build/installation order, after which the normal
+# install through setup.py picks up their wheels from the cache (TODO: note the
+# duplication here with setup.py; find a better way)
+_get_requires_for_build_wheel = main.get_requires_for_build_wheel
+def get_requires_for_build_wheel(*args, **kwds):
+    try:
+        import __pypy__, sys
+        version = sys.pypy_version_info
+        requirements = ['cppyy-backend<=1.11', 'cppyy-cling<=6.20.4']
+        if version[0] == 5:
+            if version[1] <= 9:
+                requirements = ['cppyy-backend<0.3', 'cppyy-cling<6.12']
+            elif version[1] <= 10:
+                requirements = ['cppyy-backend<0.4', 'cppyy-cling<=6.15']
+        elif version[0] == 6:
+            if version[1] <= 0:
+                requirements = ['cppyy-backend<1.1', 'cppyy-cling<=6.15']
+        elif version[0] == 7:
+            if version[1] <= 3 and version[2] <= 2:
+                requirements = ['cppyy-backend<=1.10', 'cppyy-cling<=6.18.2.3']
+    except ImportError:
+        # CPython
+        requirements = ['CPyCppyy==1.11.5', 'cppyy-backend==1.14.1', 'cppyy-cling==6.21.4']
+
+    return requirements + _get_requires_for_build_wheel(*args, **kwds)
+
+main.get_requires_for_build_wheel = get_requires_for_build_wheel

pyproject.toml

@@ -1,5 +1,4 @@
 [build-system]
-requires = ["CPyCppyy==1.11.5; python_implementation != 'PyPy'",
-            "cppyy-backend==1.14.1; python_implementation != 'PyPy'",
-            "cppyy-cling==6.21.4; python_implementation != 'PyPy'",
-            "setuptools", "wheel"]
+requires = ["setuptools", "wheel"]
+backend-path = ["installer"]
+build-backend = "cppyy_monkey_patch:main"