feat!: drop Python 3.8 support

[mkniewallner]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• If you've added or modified a feature, documentation in docs is updated

Description of changes

Python 3.8 won't be supported anymore sometimes in October.

Download stats for this month also show a pretty low usage of Python 3.8:

$ uvx pypistats python_minor deptry --this-month
┌──────────┬─────────┬───────────┐
│ category │ percent │ downloads │
├──────────┼─────────┼───────────┤
│ null     │  56.78% │   208,873 │
│ 3.12     │  21.35% │    78,538 │
│ 3.11     │  12.97% │    47,703 │
│ 3.10     │   4.84% │    17,803 │
│ 3.9      │   2.15% │     7,922 │
│ 3.8      │   1.57% │     5,785 │
│ 3.13     │   0.22% │       822 │
│ 3.7      │   0.07% │       254 │
│ 3.14     │   0.04% │       140 │
│ Total    │         │   367,840 │
└──────────┴─────────┴───────────┘

Date range: 2024-09-01 - 2024-09-27

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.1%. Comparing base (98cc5bf) to head (967286b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #874   +/-   ##
=====================================
  Coverage   92.1%   92.1%           
=====================================
  Files         37      37           
  Lines        992     992           
  Branches     175     175           
=====================================
  Hits         914     914           
  Misses        62      62           
  Partials      16      16           

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