Drop Python 3.6

[jrbourbeau]

As xhistograms dependencies, i.e. numpy, dask, and xarray, have all dropped support for Python 3.6, it probably makes sense for us to drop support here too.

[codecov]

Codecov Report

Merging #40 (9e5434b) into master (2b2b82e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #40   +/-   ##
=======================================
  Coverage   94.41%   94.41%           
=======================================
  Files           3        3           
  Lines         215      215           
  Branches       58       58           
=======================================
  Hits          203      203           
  Misses          8        8           
  Partials        4        4           

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[dougiesquire]

Agreed and thanks @jrbourbeau. Any idea why we're now regularly failing test_histogram_results_1d in a number of active PRs (#38 also)? I'm happy to try look into this if nothing springs to mind.

[jrbourbeau]

Great, thanks for reviewing @dougiesquire! I've proposed a quick fix for test_histogram_results_1d over in #41

[jrbourbeau]

Just included the quickfix from #41 here -- so CI is passing now. If we're comfortable dropping Python 3.6, then I think this PR should be good to go