Make markdown_it.common.entities.entities a dict. Remove AttrDict

[hukkin]

Hey @chrisjsewell what's your thoughts on a change like this?

One test fails because of AttDict usage in mdit-py-plugins, so would need to be fixed there first.

mdit-py-plugins uses AttrDict only this one time, and doesn't use entities at all. Not sure how much or if this would break much in MyST or elsewhere though? EDIT: MyST seems to not use entities at all, and the only use of AttrDict is env related, so should be removed regardless with markdown_it_py>=1.0.0 support.

[hukkin]

Made a PR to remove AttrDict usage from mdit-py-plugins, so that would fix the test failure here.

[hukkin]

On another note, maybe entities should be defined in common/__init__.py rather than common/entities.py ? The import path would more closely resemble JS upstream then, i.e. current from markdown_it.common.entities import entities vs from markdown_it.common import entities

[chrisjsewell]

superseded by #181