pyproject.toml: clarify build system version

[adamjstewart]

See jupyter/jupyter-packaging#120

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[codecov-commenter]

Codecov Report

Merging #222 (3f8aaf6) into master (d577120) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #222   +/-   ##
=======================================
  Coverage   76.96%   76.96%           
=======================================
  Files          28       28           
  Lines        2071     2071           
=======================================
  Hits         1594     1594           
  Misses        477      477           

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[blink1073]

Thanks! I updated it to include 1.x support.

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[adamjstewart]

Was dropping 0.9 support intentional?

[blink1073]

I was looking at jupyter/jupyter-packaging#120 when I made the change. It doesn't matter very much, since the library is only used in the isolated build environ.