Attribute handling cleanup for 1.0

[szhorvat]

This issue is to keep track of the minor attribute handling cleanup we are doing for 1.0 (not to be confused with the major overhaul planned for 2.0).

• [x] Adapt Python interface to changes, ensure there are no issues
• [ ] Adapt R interface to changes, ensure there are no issues
• [x] Update changelog

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 14 days if no further activity occurs. Thank you for your contributions.

[ntamas]

Python interface updated and adapted in igraph/python-igraph@1c423c60

[szhorvat]

@krlmlr Can you guys get started on this to verify that there are no problems before the 1.0 C core release?