GH-44651: [Python] Allow from_buffers to work with StringView on Python

[raulcd]

Rationale for this change

Currently from_buffers is not working with StringView on Python because we validate against num_buffers. This only take into account the mandatory buffers but does not take into account the variadic_spec that can be present for both string_view and binary_view

What changes are included in this PR?

Take into account whether the type contains a variadic_spec for the non-mandatory buffers and only check lower_bound number of buffers.

Are these changes tested?

Yes, I've added a couple of tests.

Are there any user-facing changes?

We are exposing a new method on the Python DataType. has_variadic_buffers which tells us whether the number of buffers expected is only lower-bounded by num_buffers.

• GitHub Issue: [Python] StringViewArray.from_buffers does not seem to work as expected #44651

[jorisvandenbossche]

Looks good!

[raulcd]

I'll be merging this as it was approved with a single minor comment about the error message formatting which has been updated

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 3 benchmarking runs that have been run so far on merge-commit aa26f28.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 27 possible false positives for unstable benchmarks that are known to sometimes produce them.