Feature or enhancement

Some of our internal headers (e.g.,pycore_ceval.h) contain#include directives inside theextern "C" { blocks. This can cause problems if system headers are included within anextern "C" { block and the header is compiled as C++ code. For a similar issue with the public headers, see#110845.

As far as I can tell, nobody has complained about this yet, but I think we might as well address this sooner rather later.

cc@ericsnowcurrently@gpshead@vstinner