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

diff --git a/Objects/call.c b/Objects/call.c
index 3b08cb2..4294a9b 100644
--- a/Objects/call.c
+++ b/Objects/call.c
@@ -499,7 +499,7 @@
         if (kwargs != NULL && PyDict_GET_SIZE(kwargs) != 0) {
             goto no_keyword_error;
         }
-        /* fall through next case */
+        /* fall through */
 
     case METH_VARARGS | METH_KEYWORDS:
     {
@@ -656,7 +656,7 @@
         if (nkwargs) {
             goto no_keyword_error;
         }
-        /* fall through next case */
+        /* fall through */
 
     case METH_VARARGS | METH_KEYWORDS:
     {
diff --git a/Objects/stringlib/codecs.h b/Objects/stringlib/codecs.h
index 43f2f32..f019d9a 100644
--- a/Objects/stringlib/codecs.h
+++ b/Objects/stringlib/codecs.h
@@ -330,7 +330,7 @@
             case _Py_ERROR_REPLACE:
                 memset(p, '?', endpos - startpos);
                 p += (endpos - startpos);
-                /* fall through the ignore handler */
+                /* fall through */
             case _Py_ERROR_IGNORE:
                 i += (endpos - startpos - 1);
                 break;
@@ -378,7 +378,7 @@
                 }
                 startpos = k;
                 assert(startpos < endpos);
-                /* fall through the default handler */
+                /* fall through */
             default:
                 rep = unicode_encode_call_errorhandler(
                       errors, &error_handler_obj, "utf-8", "surrogates not allowed",
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 0e6cb7f..f7b2aa6 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -1794,6 +1794,7 @@
 
     case SSTATE_INTERNED_IMMORTAL:
         Py_FatalError("Immortal interned string died.");
+        /* fall through */
 
     default:
         Py_FatalError("Inconsistent interned string state.");
@@ -6778,7 +6779,7 @@
             case _Py_ERROR_REPLACE:
                 memset(str, '?', collend - collstart);
                 str += (collend - collstart);
-                /* fall through ignore error handler */
+                /* fall through */
             case _Py_ERROR_IGNORE:
                 pos = collend;
                 break;
@@ -6817,7 +6818,7 @@
                     break;
                 collstart = pos;
                 assert(collstart != collend);
-                /* fallback to general error handling */
+                /* fall through */
 
             default:
                 rep = unicode_encode_call_errorhandler(errors, &error_handler_obj,