Bump version 20.12.3

[ahopkins]

Prepare for 2021-03-21 release date.

[codecov]

Codecov Report

Merging #2062 (c1d6d56) into 20.12LTS (8d86c3c) will decrease coverage by 0.092%.
The diff coverage is 100.000%.

@@              Coverage Diff               @@
##           20.12LTS     #2062       +/-   ##
==============================================
- Coverage    92.870%   92.778%   -0.092%     
==============================================
  Files            29        29               
  Lines          3268      3268               
  Branches        573       573               
==============================================
- Hits           3035      3032        -3     
- Misses          157       159        +2     
- Partials         76        77        +1     

Impacted Files	Coverage Δ
sanic/__version__.py	100.000% <100.000%> (ø)
sanic/server.py	81.614% <0.000%> (-0.563%)	⬇️

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