Bump version to 21.3.0

[rodrigc]

No description provided.

[codecov]

Codecov Report

Merging #67 (0f8be2a) into master (38af4b8) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #67   +/-   ##
=======================================
  Coverage   96.86%   96.86%           
=======================================
  Files           5        5           
  Lines         733      733           
  Branches       68       68           
=======================================
  Hits          710      710           
  Misses         11       11           
  Partials       12       12           

Impacted Files	Coverage Δ
src/incremental/_version.py	100.00% <100.00%> (ø)

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