Removing numpy casts

[mwkury]

Describe the bug
casting has been removed from modern numpy and now some features in pymcmcstat error because of this.

To Reproduce
prediction_plotter = pymc.plotting.PredictionIntervals.PredictionIntervals()
prediction_plotter.setup_prediction_interval_calculation(results, mcmc.data, model_function_for_prediction, burnin)
prediction_plotter.generate_prediction_intervals()
prediction_plotter.plot_prediction_intervals()

Expected behavior
AttributeError: module 'numpy' has no attribute 'int'.
np.int was a deprecated alias for the builtin int. To avoid this error in existing code, use int by itself. Doing this will not modify any behavior and is safe. When replacing np.int, you may wish to use e.g. np.int64 or np.int32 to specify the precision. If you wish to review your current use, check the release note link for additional information.
The aliases was originally deprecated in NumPy 1.20; for more details and guidance see the original release note at:

Additional context
I am potentially willing to help contribute to this repo and help maintain it, but I want to check in and see if there are others that are going to perform code review and confirm pull-requests.