Add compatibility to python 3.10

[MBus123]

This makes biblib work with python 3.10.

The Parser.parse() uses collections.abc per default (which is the only way in python 3.10) and only falls back to the old collections.Iterable if collections.abc does not exist.

[t-wissmann]

This PR is precisely the same as my PR #8, just with a slightly different comment.