[3.6] bpo-30923: Silence fall-through warnings included in -Wextra since gcc-7.0 (#3518)

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

(cherry picked from commit d73a960c575207539c3f9765cff26d4fff400b45)

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

(cherry picked from commit f432a3234f9f2ee09bd40be03e06bf72865ee375)

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

(cherry picked from commit 138753c1b96b5e06a5c5d409fa4cae5e2fe1108b)
diff --git a/Python/getargs.c b/Python/getargs.c
index 1381964..ed6b815 100644
--- a/Python/getargs.c
+++ b/Python/getargs.c
@@ -2260,8 +2260,8 @@
                 /* after 'e', only 's' and 't' is allowed */
                 goto err;
             format++;
-            /* explicit fallthrough to string cases */
         }
+        /* fall through */
 
     case 's': /* string */
     case 'z': /* string or None */