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/cffLib.py b/Lib/fontTools/cffLib.py
index 537fd98..e3b00c9 100644
--- a/Lib/fontTools/cffLib.py
+++ b/Lib/fontTools/cffLib.py
@@ -482,7 +482,7 @@
 				assert False, "unsupported FDSelect format: %s" % format
 		else:
 			# reading from XML. Make empty gidArray,, and leave format as passed in.
-			# format == None will result in the smallest representation being used.
+			# format is None will result in the smallest representation being used.
 			self.format = format
 			self.gidArray = []
 
@@ -514,9 +514,9 @@
 			self.charStringsAreIndexed = 0
 			self.globalSubrs = globalSubrs
 			self.private = private
-			if fdSelect != None:
+			if fdSelect is not None:
 				self.fdSelect = fdSelect
-			if fdArray!= None:
+			if fdArray is not None:
 				self.fdArray = fdArray
 	
 	def keys(self):