docs: exception syntax compatibility with Python 3

docs: exception syntax compatibility with Python 3

Since Python 2.7, there exists a new syntax to catch errors: the old way
of saying `except IOError, error` has been replaced with `except IOError
as e`. The old syntax has finally been removed in Python 3, but as we're
still using it in zziplib newer interpreters will raise a parsing error.

Convert to use the new syntax to improve compatibility with Python 3.