Updated tests to use python3.
Added wrapper3.py (python3 sample wrapper).
spec: Define ascii punctuation character separately.

That's because some ascii punctuation characters (like `<`)
are not in a "P" character class (they are mathematical symbols).
They need to be escapable so we'll treat them as "punctuation" here.
Added cmark_ctype.h with locale-independent isspace, ispunct, etc.

Otherwise cmark's behavior varies unpredictably with the locale.

`is_punctuation` in utf8.h has also been adjusted so that everything
that counts all ASCII symbol characters count as punctuation, even
though some are not in P* character classes.
Use python3 for bench in Makefile.
Fixed leakcheck for python3.