[3.6] bpo-30923: Silence fall-through warnings included in -Wextra since gcc-7.0

[vstinner]

• bpo-30923: Disable warning that has been part of -Wextra since gcc-7.0. (bpo-30923: Disable warning that has been part of -Wextra since gcc-7.0. #3142)
• bpo-30923: Silence fall-through warnings included in -Wextra since gcc-7.0. (bpo-30923: Silence fall-through warnings included in -Wextra since gc… #3157)
• bpo-31275: Small refactoring to silence a fall-through warning. (bpo-31275: Small refactoring to silence a fall-through warning. #3206)

https://bugs.python.org/issue30923

[serhiy-storchaka]

Usually we don't backport changes that just silence compiler warnings.

[skrah]

It's a double edged sword: If we don't backport, other backports become more difficult because of conflicts. Technically we should take out _codecs_iso2022.c, but I assume that @Haypo has given it another thorough look.

[vstinner]

Usually we don't backport changes that just silence compiler warnings.

IMHO the change is simple enough to justify a backport. I would like to fix these warnings to be able to detect more serious warnings.

Technically we should take out _codecs_iso2022.c, but I assume that @Haypo has given it another thorough look.

I read it and it seems safe.

[miss-islington]

Thanks, @Haypo!

[vstinner]

Thanks for the review @skrah.