factory: Remove constant variable from __slots__

In python3, if we don't initialize the variable in `__init__()` function,
it will cause error : conflict with class variable.

After check, the variable is constant, we can remove it from
`__slots__`. And there is still no `__dict__` attribute in this class in
python3.

BUG=chromium:999876
TEST=make test

Change-Id: Id89b1d48c7e64323225ceca508d611c0e6ffcf81
Reviewed-on: https://chromium-review.googlesource.com/1890461
Tested-by: Yilin Yang <kerker@chromium.org>
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Wei-Han Chen <stimim@chromium.org>
diff --git a/py/utils/pygpt.py b/py/utils/pygpt.py
index cda1564..a69d63a 100755
--- a/py/utils/pygpt.py
+++ b/py/utils/pygpt.py
@@ -71,7 +71,7 @@
 
 class UTF16StructField(StructField):
   """A field in UTF encoded string."""
-  __slots__ = ['encoding', 'max_length']
+  __slots__ = ['max_length']
   encoding = 'utf-16-le'
 
   def __init__(self, max_length, name):