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/Scripts/PackageDocumentTools/plugins/EdkPlugins/edk2/model/doxygengen_spec.py

@@ -1004,7 +1004,7 @@ class PackageDocumentAction(DoxygenAction):
 
         try:
             file = open(path, 'rb')
-        except (IOError, OSError), msg:
+        except (IOError, OSError) as msg:
             return None
 
         t = file.read()