Prepare release of v0.16.0

This MR prepares the v0.16.0 release of dantro.

Importantly, it drops support for Python 3.6, main reason being dependency issues (which would require to change test strategy for minimal dependencies) and because the writing is on the wall anyway: latest numpy and scipy versions don't support py36 anyway ... and it was initially released in 2016, so I think it's ok to not put further resources into it.

Can this MR be accepted?

• Set version number in dantro/__init__.py
  • Removed the pre-release specifier.
  • Version is now: 0.16.0
• Prepared changelog for release
  • Removed "WIP" in section heading
  • If necessary, re-ordered and cleaned-up the corresponding section
• Pipeline passes without warnings
  • If the test stage creates warnings for the *_min jobs, inspect the output log and, if necessary, adjust the lower bounds of the dependencies in setup.py.