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ARROW-7729: [Python][CI] Pin pandas version to 0.25 in the dask integration test

Additionally test agains dask's latest release not just the master revision.

Closes #6326 from kszucs/dask-pandas-pin and squashes the following commits:

b5cb40eec <Krisztián Szűcs> pin pandas depending on dask's version
083221ce9 <Krisztián Szűcs> pin pandas in the dask integration test

Authored-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>