Add compatibility to python 3.10

[t-wissmann]

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]

Any news on this?

[t-wissmann]

@aclements: is there any hope that this is merged? It should be only one click effort to merge this PR.

I'd love to use biblib also in the future, because it is the only bibtex parser that works reliably.

[t-wissmann]

There are already 6 people waiting for this change to be merged. Is there any hope for it, @aclements?