Fix python warnings in webidl binder

[kripken]

A new python build on chromium CI started to warn on these, not sure why...

[jgravelle-google]

Experiment:

>>> a = 'ab'
>>> b = 'a' + 'b'
>>> a is b
True
>>> a = 'aaaaaaaaaaaaaaaabbbbbbbbbbbbbbbb'
>>> len(a)
32
>>> b = 'a' * 16 + 'b' * 16
>>> a == b
True
>>> a is b
False
>>> 

So https://stackoverflow.com/a/2988271 essentially. This is probably fine for small strings because the python VM interns them, but in general is sketchy.

You should use == when comparing values and is when comparing identities.