[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/formatter_unicode.c b/Python/formatter_unicode.c
index a2c2b36..9192bfd 100644
--- a/Python/formatter_unicode.c
+++ b/Python/formatter_unicode.c
@@ -312,6 +312,7 @@
                 format->thousands_separators = LT_UNDER_FOUR_LOCALE;
                 break;
             }
+            /* fall through */
         default:
             invalid_comma_type(format->type);
             return 0;