Bump version to v0.7.3

[Z-Fran]

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Motivation

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Modification

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Checklist

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[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (995a5e9) 74.81% compared to head (d297e6d) 74.81%.

❗ Current head d297e6d differs from pull request most recent head 7982da7. Consider uploading reports for the commit 7982da7 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #506   +/-   ##
=======================================
  Coverage   74.81%   74.81%           
=======================================
  Files         145      145           
  Lines       11015    11015           
  Branches     2097     2097           
=======================================
  Hits         8241     8241           
  Misses       2233     2233           
  Partials      541      541           

Flag	Coverage Δ
unittests	74.81% <100.00%> (ø)

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Impacted Files	Coverage Δ
mmgen/version.py	58.33% <100.00%> (ø)

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