I understand where you are coming from and I do agree, but this is something bigger than just what we can do in an issue, as it spawns multiple other places (again, many files fail with -pedantic). Is not a titanic task but is also not straightforward. My recommendation is to send a brief email to python-dev about these concerns to gather some feedback from the community, as this issue is probably not going to have all the attention that this would otherwise require.

For example, there are several pieces of infrastructure that use GNU extensions already by default when compiled with clang and gcc (like computed gotos), so activating -pedantic will invalidate those simple checks.