Expose `arch` in Python bindings

[alebastr]

I'm trying to write a Python code that traverses the Fedora comps and writes a list of packages for the groups, grouping the packages by supported architectures (an update for comps-sync.py in Fedora's workstation-ostree-config).

The data we need, <packagereq arch="..." and <groupid arch="..." attributes are not exposed through the Python bindings. Would it be possible to add support for that?

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I've tried to write the necessary bindings myself, but I lack the python C API knowledge required to debug a few remaining crashes. I can send a draft PR, but IMO it would be better if someone familiar with the codebase could work on that.

[AdamWill]

You don't really need this. See my comps-sync PR. It also doesn't really match the way this was designed to work. The design is that you query filters by arch if you want results filtered by arch. A query that's not filtered is implicitly a query for all arches. This approach happens to make things slightly awkward for the way comps-sync.py wants to work, but I'm not sure that exposing this attribute in the results of the search is really the 'right thing' to do, given libcomps' actual design here.