Suppress -Wregister gcc warnings in Python.h

Such as:
/usr/include/python2.7/unicodeobject.h:534:24: warning: ISO C++17 does not allow ‘register’ storage class specifier [-Wregister]

Closes #2227

Lib/python/embed.i

@@ -32,8 +32,20 @@ As far as I know, this module is C++ safe.
 #ifndef SWIG_NO_PY_SSIZE_T_CLEAN
 #define PY_SSIZE_T_CLEAN
 #endif
+
+#if __GNUC__ >= 7
+#pragma GCC diagnostic push
+#if defined(__cplusplus) && __cplusplus >=201703L
+#pragma GCC diagnostic ignored "-Wregister" /* For python-2.7 headers that use register */
+#endif
+#endif
+
 #include <Python.h>
 
+#if __GNUC__ >= 7
+#pragma GCC diagnostic pop
+#endif
+
 #ifdef __cplusplus
 extern "C"
 #endif

Lib/python/pyruntime.swg

@@ -14,6 +14,14 @@
 #ifndef SWIG_NO_PY_SSIZE_T_CLEAN
 #define PY_SSIZE_T_CLEAN
 #endif
+
+#if __GNUC__ >= 7
+#pragma GCC diagnostic push
+#if defined(__cplusplus) && __cplusplus >=201703L
+#pragma GCC diagnostic ignored "-Wregister" /* For python-2.7 headers that use register */
+#endif
+#endif
+
 #if defined(_DEBUG) && defined(SWIG_PYTHON_INTERPRETER_NO_DEBUG)
 /* Use debug wrappers with the Python release dll */
 # undef _DEBUG
@@ -22,6 +30,10 @@
 #else
 # include <Python.h>
 #endif
+
+#if __GNUC__ >= 7
+#pragma GCC diagnostic pop
+#endif
 %}
 
 %insert(runtime) "swigrun.swg";         /* SWIG API */