release(hansbug): use version 0.2.1

[HansBug]

Release version of 0.2.1.

[codecov]

Codecov Report

Merging #4 (716b4b9) into main (fe5f681) will increase coverage by 0.30%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main       #4      +/-   ##
==========================================
+ Coverage   97.54%   97.84%   +0.30%     
==========================================
  Files          32       32              
  Lines        1545     1576      +31     
==========================================
+ Hits         1507     1542      +35     
+ Misses         38       34       -4     

Flag	Coverage Δ
unittests	97.84% <100.00%> (+0.30%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
treetensor/common/module.py	100.00% <100.00%> (ø)
treetensor/common/wrappers.py	100.00% <100.00%> (ø)
treetensor/config/meta.py	100.00% <100.00%> (ø)
treetensor/numpy/array.py	96.80% <100.00%> (+0.46%)	⬆️
treetensor/numpy/funcs.py	100.00% <100.00%> (ø)
treetensor/common/trees.py	88.52% <0.00%> (+3.27%)	⬆️
treetensor/numpy/__init__.py	88.23% <0.00%> (+5.88%)	⬆️

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