Update pytest-cov requirement from <5 to <6 in /by-dataframe/dask

Updates the requirements on [pytest-cov](https://github.com/pytest-dev/pytest-cov) to permit the latest version.
- [Changelog](https://github.com/pytest-dev/pytest-cov/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-cov@v1.0...v5.0.0)

---
updated-dependencies:
- dependency-name: pytest-cov
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

by-dataframe/dask/requirements-dev.txt

@@ -1,2 +1,2 @@
 pytest<9
-pytest-cov<5
+pytest-cov<6