BaseTools: Handle the bytes and str difference

Deal with bytes and str is different, remove the unicode()
Using utcfromtimestamp instead of fromtimestamp.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Yunhua Feng <yunhuax.feng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

BaseTools/Source/Python/AutoGen/ValidCheckingInfoObject.py

@@ -41,7 +41,7 @@ class VAR_CHECK_PCD_VARIABLE_TAB_CONTAINER(object):
             os.mkdir(dest)
         BinFileName = "PcdVarCheck.bin"
         BinFilePath = os.path.join(dest, BinFileName)
-        Buffer = ''
+        Buffer = bytearray()
         index = 0
         for var_check_tab in self.var_check_info:
             index += 1