Use python sets

[blag]

Instead of

for (loop):
    if (condition):
        continue

simply use Python sets:

for attr in set(attrs.keys()).intersection(set(self.attr_val_is_uri)):
    ...

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/1615

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[gsnedders]

Can you have each branch based on upstream master, so you don't have massive overlap between PRs? Note that #110 makes large changes to the sanitizer — including changing the lists to sets.

[blag]

Sorry I'm still new to using Github's pull requests. This is fixed.

And yes, this is superseded by #110.

[gsnedders]

Per above, superseded.