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

從 docs/starting/which-python.rst 中的 CPython 區段開始，查看連結的 PyPy、JyNI 和 IronPython 擴充功能文件。確認每個實作目前對 C 擴充功能的支援範圍，然後更新該敘述，使其準確描述可用的選項。