unix/mpconfigport.h: Enable MICROPY_PY_DELATTR_SETATTR.

This is a generally useful feature and because it's part of the object
model it cannot be added at runtime by some loadable Python code, so enable
it on the standard unix build.

ports/unix/mpconfigport.h

@@ -87,6 +87,7 @@
 #define MICROPY_VFS_POSIX_FILE      (1)
 #define MICROPY_PY_FUNCTION_ATTRS   (1)
 #define MICROPY_PY_DESCRIPTORS      (1)
+#define MICROPY_PY_DELATTR_SETATTR  (1)
 #define MICROPY_PY_BUILTINS_STR_UNICODE (1)
 #define MICROPY_PY_BUILTINS_STR_CENTER (1)
 #define MICROPY_PY_BUILTINS_STR_PARTITION (1)