feat: switch from import assertions (assert) to import attributes (with)

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🚀 Feature: Use 'with' keyword, not 'assert', for .json imports #20
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

💖

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.12%. Comparing base (a9e4c53) to head (e73b29e).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
- Coverage   97.40%   97.12%   -0.29%     
==========================================
  Files           8       13       +5     
  Lines         270      278       +8     
  Branches        8       30      +22     
==========================================
+ Hits          263      270       +7     
- Misses          7        8       +1     

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