python: Use the unicode_escape codec

python: Use the unicode_escape codec

Python 2 had string_escape and unicode_escape codecs. Python 3 only has
the latter. These work the same as far as we're concerned, so let's use
the future-proof one.

However, the reste of the code expects unicode strings, so we need to
decode them again.

Signed-off-by: Mathieu Bridon <bochecha@daitauha.fr>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>