chore: use new mypy config `extra_checks`

[mr-cal]

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---

Resolves the following deprecation warning:

lint-mypy: commands[0]> mypy --install-types --non-interactive .
Warning: --strict-concatenate is deprecated; use --extra-checks instead

Source: https://github.com/canonical/craft-providers/commit/0b3f047763ce2e3f683f726c3548442242781aa2

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (46e9d22) 100.00% compared to head (00c4e83) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #158 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 1 1 Lines 10 10 Branches 2 2 ========================================= Hits 10 10 ```

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