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

Author: Stefan Krah

Modules/_decimal/libmpdec/io.c

@@ -45,6 +45,12 @@
    PEP-3101 formatting for numeric types. */
 
 
+/* Disable warning that is part of -Wextra since gcc 7.0. */
+#if defined(__GNUC__) && !defined(__INTEL_COMPILER) && __GNUC__ >= 7
+  #pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
+#endif
+
+
 /*
  * Work around the behavior of tolower() and strcasecmp() in certain
  * locales. For example, in tr_TR.utf8: