Windows specific int comparison error

[Caleb-Carr]

In pdb_utils.py, the following line in def reassign_b_factor raises an error when run on a windows machine because windows handles int differently than unix systems

if df[site_col].dtype != int:
        raise ValueError("function currently requires `site_col` to be int")

This is explained in the following post: https://stackoverflow.com/questions/64901822/why-do-pandas-integer-dtypes-not-behave-the-same-on-unix-and-windows

[jbloom]

@Caleb-Carr, you can add a pull request to fix this (and any other Windows specific issues) if you want. Otherwise I am probably not going to do it though as only use Linux / Mac and that is all we are using for testing.