Bump version to 0.31.0

[A5rocks]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (439de50) to head (8f7f51c).
⚠️ Report is 59 commits behind head on main.

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3328   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             125          125           
  Lines           19497        19497           
  Branches         1334         1334           
===============================================
  Hits            19497        19497           

Files with missing lines	Coverage Δ
src/trio/_version.py	100.00000% <100.00000%> (ø)

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