Allow 5.1 of PyYAML

[dstufft]

Closes #4230

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[codecov-io]

Codecov Report

Merging #4231 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #4231   +/-   ##
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  Coverage    94.41%   94.41%           
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  Files          188      188           
  Lines        14151    14151           
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  Hits         13360    13360           
  Misses         791      791

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[asottile]

I've had this PR for months: #4015!

[dstufft]

Yea, this is an adaption of that PR that cleans up the logic in setup.py so that it isn't dependent on the ordering of requires anymore.

[asottile]

I'm a bit miffed, not going to lie -- that could have been a comment and an easy fix up if that truly was a blocking issue instead of ignoring my PR for months and then merging essentially an identical PR.

[CumpsD]

Is there any way to easily get awscli to install if there is PyYAML 5.1.1 installed system wide (via Arch python-yaml dist package)?