Update version to 3.1.0-RC0

[mwestphal]

#2108

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.91%. Comparing base (8d04b92) to head (c77fefe).
Report is 183 commits behind head on release.

Additional details and impacted files

@@             Coverage Diff             @@
##           release    #2111      +/-   ##
===========================================
+ Coverage    95.67%   95.91%   +0.23%     
===========================================
  Files          125      127       +2     
  Lines         9947    11095    +1148     
===========================================
+ Hits          9517    10642    +1125     
- Misses         430      453      +23     

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