* bpo-30923: Disable warning that has been part of -Wextra since gcc-7.0. (#3142)

(cherry picked from commit d73a960c)

* bpo-30923: Silence fall-through warnings included in -Wextra since gcc-7.0. (#3157)

(cherry picked from commit f432a323)

* bpo-31275: Small refactoring to silence a fall-through warning. (#3206)

(cherry picked from commit 138753c1)