CLN: Use new-style classes instead of old-style classes
Noticed some [lgtm.com alerts](https://lgtm.com/projects/g/pydata/pandas/snapshot/f85a69a8fdaed49747c4db61f5177a296cf290eb/files/scripts/validate_docstrings.py?sort=name&dir=ASC&mode=heatmap&showExcluded=false#L108) about the [`@property` decorator not working in old-style classes](https://lgtm.com/rules/10030086/) in `scripts/validate_docstrings.py`.  I don't think this actually causes any undesired behavior in that script though.

Figured this would be a good time to enforce new-style classes throughout the codebase:
  - Converted old-style classes to new-style classes
      - Basically just inserted `(object)`.
  - Added a check to `lint.sh` for old-style classes, since `flake8` doesn't look to be catching it.
      - Maybe there's a way to enforce this in `flake8` instead?
  - Didn't hit the Cython code; not exactly sure what the new-style vs. old-style conventions are there.