Make github workflow which would ensure that PR against master would have everything from maint merged

[yarikoptic]

I think we should retain the habit of releasing minor (or eventually next major) versions from master while patch fixes from maint (we might even keep releasing patch fixes for the prior minor I guess... but probably we shouldn't).

This way we can 'stage' for next minor release while releasing patch releases without bottleneck/force to release miors.

With this PR I want to make sure that when we do release next minor, we do include all fixes from maint.

As only pertinent to master branch -- I positioned it against master. Also would make it easier to check

PR checklist

• Provide an overview of the changes you're making and explain why you're proposing them.
• Create a changelog snippet (add the CHANGELOG-missing label to this pull request in order to have a snippet generated from its title;
  or use scriv create locally and include the generated file in the pull request, see scriv).
• Sophisticate workflow to trigger also on any other push to master or maint so it stays "up to date" even if no changes to PR happen -- decided to postpone for later... would need more work.

edit: I canceled appveyor since unrelated

[yarikoptic]

ok, seems good. Let me merge now

• Raise exception if an annex remote process without console tries to interact with the user, e.g. prompt for a password #7578
  into maint ;)

[codeclimate]

Code Climate has analyzed commit 5b3ec33 and detected 0 issues on this pull request.

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[yarikoptic]

I clarified wording in the changelog entry.
As it is just a "safe guard" for releases I think it would be ok to merge -- which I will do in a few days unless there is a push back.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.67%. Comparing base (864dc4a) to head (5b3ec33).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7590      +/-   ##
==========================================
- Coverage   91.35%   90.67%   -0.68%     
==========================================
  Files         325      325              
  Lines       43442    43438       -4     
  Branches     5778        0    -5778     
==========================================
- Hits        39685    39389     -296     
- Misses       3741     4049     +308     
+ Partials       16        0      -16     

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