Update mcse_sd calculation to not use normality assumption.

[ahartikainen]

Fixes #2005

See https://github.com/stan-dev/posterior/pull/233/files

Description

Checklist

• [ ] Follows official PR format
• [ ] Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• [ ] New features are properly documented (with an example if appropriate)?
• [ ] Includes new or updated tests to cover the new feature
• [ ] Code style correct (follows pylint and black guidelines)
• [ ] Changes are listed in changelog

[OriolAbril]

It looks like tests are running against posterior 0.1.2, can this be updated to latest posterior?