Use "is None" instead of "== None"

The latter hits the __eq__ method and can fail because we now
do not allow comparing objects of different types.

For example, was failing subsetting Andika-R.ttf.
diff --git a/Lib/fontTools/ttLib/sfnt.py b/Lib/fontTools/ttLib/sfnt.py
index 4740376..6291510 100644
--- a/Lib/fontTools/ttLib/sfnt.py
+++ b/Lib/fontTools/ttLib/sfnt.py
@@ -189,7 +189,7 @@
 				self.totalSfntSize += (entry.origLength + 3) & ~3
 
 			data = self.flavorData if self.flavorData else WOFFFlavorData()
-			if data.majorVersion != None and data.minorVersion != None:
+			if data.majorVersion is not None and data.minorVersion is not None:
 				self.majorVersion = data.majorVersion
 				self.minorVersion = data.minorVersion
 			else: