Prepare release 3.1.0

[khaeru]

Per the release procedure:

• Ensure all continuous integration checks pass.
• On https://readthedocs.com, enable the build of the RC branch and ensure the docs build correctly.
• Test publishing using TestPyPI.

[codecov]

Codecov Report

Merging #366 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #366   +/-   ##
=======================================
  Coverage   96.82%   96.82%           
=======================================
  Files          46       46           
  Lines        5255     5255           
=======================================
  Hits         5088     5088           
  Misses        167      167           

Impacted Files	Coverage Δ
ixmp/core.py	94.81% <100.00%> (ø)

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