Update dependency pytest-cov to v6 (#322)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [pytest-cov](https://github.com/pytest-dev/pytest-cov)
([changelog](https://pytest-cov.readthedocs.io/en/latest/changelog.html))
| project.optional-dependencies | major | `>=5.0.0,<6` -> `>=6,<7` |

---

### Release Notes

<details>
<summary>pytest-dev/pytest-cov (pytest-cov)</summary>

###
[`v6.0.0`](https://github.com/pytest-dev/pytest-cov/blob/HEAD/CHANGELOG.rst#600-2024-10-29)

[Compare
Source](https://github.com/pytest-dev/pytest-cov/compare/v5.0.0...v6.0.0)

- Updated various documentation inaccuracies, especially on subprocess
handling.
- Changed fail under checks to use the precision set in the coverage
configuration.
    Now it will perform the check just like `coverage report` would.
- Added a `--cov-precision` cli option that can override the value set
in your coverage configuration.
-   Dropped support for now EOL Python 3.8.

</details>

---

### Configuration

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are satisfied.

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rebase/retry checkbox.

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again.

---

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---

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pyproject.toml

@@ -25,7 +25,7 @@ optional-dependencies.dev = [
     "ipdb>=0.13.13,<1",
     "pandas-stubs>=2.2.3,<3",
     "mypy>=1.11.2,<2",
-    "pytest-cov>=5.0.0,<6",
+    "pytest-cov>=6,<7",
     "pytest-random-order>=1.1.1,<2",
     "pytest-xdist>=3.6.1,<4",
     "pytest>=8.3.3,<9",