STYLE: Add additional flake8 plugins
xref https://github.com/pandas-dev/pandas/pull/20563#issuecomment-377648415

Might be nice to add some additional `flake8` plugins to check various things:
  - Enforce using new-style classes?
     - The [hacking](https://github.com/openstack-dev/hacking) package looks to do this (cc @mroeschke)
     - Added a related check to `lint.sh` in #20563, which can be removed if this is added
  - Enforce PEP8 naming conventions for classes?
    - Doesn't look like this is currently enforced

Any other ideas are welcome.