Fix deprecated is_categorical calls

[caneff]

This makes frames_test.py::DeferredFrameTest::test_categorical_groupby fail for AssertionError: NonParallelOperation not raised. Getting rid of the nonparallel=True option in those tests fails with

E       AssertionError: Expression does not preserve partitioning!
E                       Expression: ComputedExpression[post_combine_sum_DataFrame_140482246047888]
E                       Requires: Index
E                       Preserves: Arbitrary
E                       Input partitioning: Index
E                       Expected output partitioning: Index

[caneff]

R: @tvalentyn

[github-actions]

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