Clean up dependencies

[bhazelton]

Six is listed as a dependency, but it doesn't appear to actually be used anywhere in the project and doesn't make sense given that it appears this repo only supports python 3+.

Also, no versions are specified for numpy or scipy. It'd be helpful to specify the minimum version of each that is expected to work with this package.

[tyler-a-cox]

Thanks for pointing this out! I think six is leftover from when we split hera_filters from uvtools. I'll go ahead and remove that and update the minimum versions of scipy and numpy .