Fix variadic tuple substitution in schema

[wyfo]

No description provided.

[codecov-commenter]

Codecov Report

Merging #344 (1d6ec8b) into master (af1bf4a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #344   +/-   ##
=======================================
  Coverage   86.13%   86.14%           
=======================================
  Files          67       67           
  Lines        5879     5882    +3     
  Branches     1218     1219    +1     
=======================================
+ Hits         5064     5067    +3     
  Misses        596      596           
  Partials      219      219           

Impacted Files	Coverage Δ
apischema/utils.py	87.61% <100.00%> (+0.17%)	⬆️

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