At Modules/_io/textio.c:285, one argument to _PyIncrementalNewlineDecoder_decode is named "_self." The name "_self," however is a keyword on older Microsoft C compilers and certain other compilers attempting to maintain compatibility with Microsoft. Renaming the argument to "myself" would fix the problem with certain compilers (Microsoft C, Open Watcom, possibly more). Please note that this is not an issue with MSVC.
I don't feel strongly about this. However, ISTM that we work reasonably hard to work with the vagaries of *nix toolchains so I don't see why an unintrusive change like this shouldn't go in to support some corner cases on the Windows front.
That was sloppy of me to not check other modules. I hadn't encountered them in my build yet. Here's a patch for Modules/_ctypes/_ctypes.c and Modules/_ctypes/callbacks.c. The arguments were changed similarly to "myself."