Drop python 3.6

[ericpre]

This is based on #2797 to show that the test failure works fine and also because it includes the fixes for the latest version of h5py (3.5).
Only the last two commits are relevant, once #2797 has been merged, I will rebase this branch if necessary and the changes will mcuh more simple.

Progress of the PR

• Drop support for python 3.6,
• [n/a] update docstring (if appropriate),
• [n/a] update user guide (if appropriate),
• add an changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting changelog entry in the readthedocs doc build of this PR (link in github checks)
• add tests,
• ready for review.

[codecov]

Codecov Report

Merging #2839 (bd5616c) into RELEASE_next_minor (707e1f9) will not change coverage.
The diff coverage is n/a.

@@                 Coverage Diff                 @@
##           RELEASE_next_minor    #2839   +/-   ##
===================================================
  Coverage               77.11%   77.11%           
===================================================
  Files                     206      206           
  Lines                   31600    31600           
  Branches                 6918     6918           
===================================================
  Hits                    24367    24367           
  Misses                   5482     5482           
  Partials                 1751     1751           

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