The sea should be marked as C considering that’s what you’ll discover when you get deep into it.
https://en.m.wikipedia.org/wiki/PyPy
Their greatest mistake was not naming it Ouroboros.
The reason C becomes relevant to Python users isn’t typically because the interpreter is written in C, but because so many important libraries (especially numpy) are implemented in C.
Extension modules are implemented in C because the interpreter is written in C. If it were written in another language, folks would write extension modules for that language instead. Also, it would be less relevant if people used portable C bindings like cffi, which are portable to PyPy and other interpreters… but they don’t.