`attrdict` is still used in one place.

[chahak13]

Describe the bug
attrdict hasn't been under development since quite a while now. agavepy/util.py has an implementation of AttrDict too. I think that should be used in agavepy/settings/__init__.py instead of from attrdict import AttrDict. That should remove the attrdict dependency and also allow compatibility with newer releases of python (>3.6).

To Reproduce
Install agavepy in any version of python >3.6 (I tried with 3.10) and try running import agavepy

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/chahak/Documents/agavepy/agavepy/__init__.py", line 1, in <module>
    from .agave import Agave
  File "/home/chahak/Documents/agavepy/agavepy/agave.py", line 17, in <module>
    from agavepy import settings
  File "/home/chahak/Documents/agavepy/agavepy/settings/__init__.py", line 14, in <module>
    from attrdict import AttrDict  # noqa
  File "/home/chahak/.local/share/virtualenvs/agavepy-GvSzWhBC/lib/python3.10/site-packages/attrdict/__init__.py", line 5, in <module>
    from attrdict.mapping import AttrMap
  File "/home/chahak/.local/share/virtualenvs/agavepy-GvSzWhBC/lib/python3.10/site-packages/attrdict/mapping.py", line 4, in <module>
    from collections import Mapping
ImportError: cannot import name 'Mapping' from 'collections' (/usr/lib/python3.10/collections/__init__.py)

I tried a simple fix by changing that line to from ..util import AttrDict and it seemed to work fine.

Desktop (please complete the following information):

• OS: ArchLinux