> CPython is written on C and provides C API.

If you look at the title of https://docs.python.org/2/extending/extending.html clearly C++ extensions are also supported.

> Even if change the signature of one function, this will not help much, because a lot of other functions require "char *" instead of "const char *".

I don't know which functions you mean, I only encountered this problem for PyErr_BadInternalCall().

I should also add that the problem cannot be avoided or worked around for PyErr_BadInternalCall() because it's a CPython header itself which does an illegal conversion of a string constant to char*.

> This will lead to incompatibility of your extension with older bugfix releases.

I don't see why there would be ABI incompatibility. My patch is only adding "const" in the call of a C function.