BaseTools: Refactor python except statements

Convert "except ... ," to "except ... as" to be compatible with python3.
Based on "futurize -f lib2to3.fixes.fix_except"

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: Yonghong Zhu <yonghong.zhu@intel.com>

BaseTools/Source/Python/AutoGen/GenDepex.py

@@ -449,7 +449,7 @@ def Main():
                     os.utime(Option.OutputFile, None)
         else:
             Dpx.Generate()
-    except BaseException, X:
+    except BaseException as X:
         EdkLogger.quiet("")
         if Option is not None and Option.debug is not None:
             EdkLogger.quiet(traceback.format_exc())