Docstrings for functions/methods in PyPy/GraalPy

[skirpichev]

GraalPy supports __doc__ for callables, but doesn't strip out special CPython syntax for text signatures (see oracle/graalpython#593).

PyPy supports __doc__ for module functions, though see pypy/pypy#5368. But not for methods in the cpyext's types:

Python 3.11.13 (413c9b7f57f5, Jul 03 2025, 18:03:56)
[PyPy 7.3.20 with GCC 10.2.1 20210130 (Red Hat 10.2.1-11)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>> from gmp import *
meson-python: building python-gmp: /home/sk/src/python-gmp/.venv/pypy3.11-7.3.20/bin/ninja --verbose
[1/2] cc -Igmp.pypy311-pp73-x86_64-linux-gnu.so.p -I. -I../.. -I/usr/local/include -I/usr/local/include/pypy3.11 -fvisibility=hidden -fdiagnostics-color=always -DNDEBUG -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -std=c17 -O3 -Wall -Wpedantic -Werror -Wconversion -fPIC -MD -MQ gmp.pypy311-pp73-x86_64-linux-gnu.so.p/gmp.c.o -MF gmp.pypy311-pp73-x86_64-linux-gnu.so.p/gmp.c.o.d -o gmp.pypy311-pp73-x86_64-linux-gnu.so.p/gmp.c.o -c ../../gmp.c
[2/2] cc  -o gmp.pypy311-pp73-x86_64-linux-gnu.so gmp.pypy311-pp73-x86_64-linux-gnu.so.p/fmt.c.o gmp.pypy311-pp73-x86_64-linux-gnu.so.p/gmp.c.o gmp.pypy311-pp73-x86_64-linux-gnu.so.p/utils.c.o -Wl,--as-needed -Wl,--allow-shlib-undefined -Wl,-O1 -shared -fPIC -Wall -Wpedantic -Werror -Wconversion -Wl,-rpath,/usr/local/lib /usr/local/lib/libzz.so
>>>> mpz.is_integer.__doc__ is None
True