Unify indent size, two in toml file

[nzw0301]

Motivation

I was a bit uncomfortable using different indent sizes in the toml file when I checked the file. Even there is no style guide in toml (toml-lang/toml#608).

Description of the changes

Replace four space indent with two by following https://packaging.python.org/en/latest/guides/writing-pyproject-toml/.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.59%. Comparing base (855692a) to head (0791891).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5271      +/-   ##
==========================================
+ Coverage   89.57%   89.59%   +0.01%     
==========================================
  Files         209      209              
  Lines       13097    13097              
==========================================
+ Hits        11732    11734       +2     
+ Misses       1365     1363       -2     

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[y0z]

LGTM.