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_iostream.py

@@ -6,8 +6,14 @@ from io import StringIO
 
 import pytest
 
+import env  # noqa: F401
 from pybind11_tests import iostream as m
 
+pytestmark = pytest.mark.skipif(
+    "env.GRAALPY",
+    reason="Delayed prints from finalizers from other tests can end up in the output",
+)
+
 
 def test_captured(capsys):
     msg = "I've been redirected to Python, I hope!"