Non-CPython support for C-extensions

From https://github.com/realpython/python-guide/blob/master/docs/starting/which-python.rst#cpython :

If you are writing open-source Python code and want to reach the widest possible audience, targeting CPython is best. To use packages which rely on C extensions to function, CPython is your only implementation option.

PyPy now supports (some/all?) C extensions
http://doc.pypy.org/en/latest/extending.html

Jython supports C extensions through JyNI:
https://github.com/Stewori/JyNI

IronPython supports C extensions now?
https://ironpython-test.readthedocs.io/en/latest/extending/extending.html

从 docs/starting/which-python.rst 中的 CPython 部分开始，查看链接的 PyPy、JyNI 和 IronPython 扩展文档。确认每个实现当前对 C 扩展的支持范围，然后更新该表述，使其准确描述可用选项。