BaseTools: Do not call sys.setdefaultencoding with python 3

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2301

This interface was a originally a no-op in python 3, and now
is fully removed causing a build warning on macOS (Darwin).

Signed-off-by: Vitaly Cheptsov <vit9696@protonmail.com>>
Acked-by: Bob Feng <bob.c.feng@intel.com>

BaseTools/Source/Python/sitecustomize.py

@@ -7,7 +7,7 @@
 import sys
 import locale
 
-if sys.platform == "darwin":
+if sys.platform == "darwin" and sys.version_info[0] < 3:
   DefaultLocal = locale.getdefaultlocale()[1]
   if DefaultLocal is None:
     DefaultLocal = 'UTF8'