Use importlib_resources to read schema.json

[jtpio]

This should help fix an issue noticed when trying to release the releaser.

Example run: https://github.com/jtpio/jupyter_releaser/runs/3542239326

[codecov-commenter]

Codecov Report

Merging #138 (78be2b7) into master (a5be6ea) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #138   +/-   ##
=======================================
  Coverage   85.50%   85.50%           
=======================================
  Files          17       17           
  Lines        2104     2104           
  Branches      253      253           
=======================================
  Hits         1799     1799           
  Misses        213      213           
  Partials       92       92           

Impacted Files	Coverage Δ
jupyter_releaser/util.py	69.27% <100.00%> (ø)

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[blink1073]

Thank you!