Bump version to 0.28.0

[A5rocks]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00000%. Comparing base (8a28c55) to head (2c466cf).
Report is 3 commits behind head on main.

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3172   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             124          124           
  Lines           18458        18458           
  Branches         1215         1215           
===============================================
  Hits            18458        18458           

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

[A5rocks]

Codecov strict sucks...