tools: Avoid Unicode error in kconfiglib while opening files

This fix is for IDF v4.0 and earlier versions. The upstream kconfiglib
included since IDF v4.1 doesn't have this issue.

Closes https://github.com/espressif/esp-idf/issues/4977

tools/kconfig_new/kconfiglib.py

@@ -1125,7 +1125,7 @@ class Kconfig(object):
         was set when the configuration was loaded.
         """
         try:
-            return open(filename)
+            return open(filename) if sys.version_info[0] < 3 else open(filename, encoding='utf-8')
         except IOError as e:
             if not os.path.isabs(filename) and self.srctree is not None:
                 filename = os.path.join(self.srctree, filename)