BaseTools: Use absolute import in AutoGen

Based on "futurize -f libfuturize.fixes.fix_absolute_import

Contributed-under: TianoCore Contribution Agreement 1.1
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>

BaseTools/Source/Python/AutoGen/StrGather.py

@@ -14,10 +14,11 @@
 ##
 # Import Modules
 #
+from __future__ import absolute_import
 import re
 import Common.EdkLogger as EdkLogger
 from Common.BuildToolError import *
-from UniClassObject import *
+from .UniClassObject import *
 from io import BytesIO
 from struct import pack, unpack
 from Common.LongFilePathSupport import OpenLongFilePath as open