Add support for GraalPy (#5380)

* Initial support for GraalPy

* Mark tests that currently fail on GraalPy with xfail

* Add graalpy to CI

* Limit test deps on graalpy to available binary wheels

* Skip cmake test installed_function on GraalPy

CMake won't find libpython on GraalPy, it either fails or silently picks
CPython's libpython.

* Factor out setting function docstrings into a macro

* Try to narrow down skipped tests

tests/test_cpp_conduit.py

@@ -7,6 +7,7 @@ import exo_planet_pybind11
 import home_planet_very_lonely_traveler
 import pytest
 
+import env
 from pybind11_tests import cpp_conduit as home_planet
 
 
@@ -27,7 +28,10 @@ def test_call_cpp_conduit_success():
         home_planet.cpp_type_info_capsule_Traveler,
         b"raw_pointer_ephemeral",
     )
-    assert cap.__class__.__name__ == "PyCapsule"
+    assert cap.__class__.__name__ == "PyCapsule" or (
+        # Note: this will become unnecessary in the next GraalPy release
+        env.GRAALPY and cap.__class__.__name__ == "capsule"
+    )
 
 
 def test_call_cpp_conduit_platform_abi_id_mismatch():