Refer to EOF by its constant everywhere (html5lib#388)

html5lib/_inputstream.py

@@ -367,7 +367,7 @@ def charsUntil(self, characters, opposite=False):
  def unget(self, char):
  # Only one character is allowed to be ungotten at once - it must
  # be consumed again before any further call to unget
- if char is not None:
+ if char is not EOF:
  if self.chunkOffset == 0:
  # unget is called quite rarely, so it's a good idea to do
  # more work here if it saves a bit of work in the frequently