sfnt: must import SimpleNamespace

Thanks Zachary for reporting this
https://github.com/fonttools/fonttools/commit/7ca42f6623dd748c913dde5ada528f8b27f96ac9#commitcomment-39299386

Amazingly we have no tests for reading TTCs
diff --git a/Lib/fontTools/ttLib/sfnt.py b/Lib/fontTools/ttLib/sfnt.py
index 5996ec3..2f3b669 100644
--- a/Lib/fontTools/ttLib/sfnt.py
+++ b/Lib/fontTools/ttLib/sfnt.py
@@ -13,6 +13,7 @@
 """
 
 from io import BytesIO
+from types import SimpleNamespace
 from fontTools.misc.py23 import Tag
 from fontTools.misc import sstruct
 from fontTools.ttLib import TTLibError