exports: Just check for the visibility attribute.

The comment alludes to releases that are no longer relevant. Ubuntu
20.04 LTS is no longer supported, but even it has gcc 9. CentOS is
in a much different state, but even the spiritual successor
rockylinux 8.9 (out of its support window as well) has gcc 8.

In short the compilers which do not support the has_attribute
directive are no longer themselves supported.

Author: tfogal

Include/exports.h

@@ -35,14 +35,7 @@
         #define Py_LOCAL_SYMBOL
     #endif
 #else
-/*
- * If we only ever used gcc >= 5, we could use __has_attribute(visibility)
- * as a cross-platform way to determine if visibility is supported. However,
- * we may still need to support gcc >= 4, as some Ubuntu LTS and Centos versions
- * have 4 < gcc < 5.
- */
-    #if (defined(__GNUC__) && (__GNUC__ >= 4)) ||\
-        (defined(__clang__) && _Py__has_attribute(visibility))
+    #if _Py__has_attribute(visibility)
         #define Py_IMPORTED_SYMBOL __attribute__ ((visibility ("default")))
         #define Py_EXPORTED_SYMBOL __attribute__ ((visibility ("default")))
         #define Py_LOCAL_SYMBOL  __attribute__ ((visibility ("hidden")))