test[python]: Fix type indexing py3.7 test error

[zundertj]

In #4058, I enforce strict type equality checks in mypy. Most cases can be fixed with typing.cast. For generic types such as tuple, we cannot use this yet, because py3.7 does not allow this, and thus we need a type: ignore. This one case of using the cast slipped through, fixing this here.

As noted by #4413 (comment)

[codecov-commenter]

Codecov Report

Merging #4416 (bcea7df) into master (beb802a) will decrease coverage by 4.72%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4416      +/-   ##
==========================================
- Coverage   78.98%   74.26%   -4.73%     
==========================================
  Files         483      483              
  Lines       77094    77111      +17     
==========================================
- Hits        60895    57266    -3629     
- Misses      16199    19845    +3646     

Impacted Files	Coverage Δ
...ars-arrow/src/kernels/rolling/no_nulls/variance.rs	88.55% <100.00%> (+0.23%)	⬆️
polars/polars-lazy/src/logical_plan/aexpr.rs	84.21% <100.00%> (+1.06%)	⬆️
polars/polars-lazy/src/physical_plan/planner/lp.rs	83.16% <100.00%> (-4.93%)	⬇️
polars/polars-io/src/tests.rs	0.00% <0.00%> (-100.00%)	⬇️
polars/polars-lazy/src/lib.rs	0.00% <0.00%> (-100.00%)	⬇️
polars/polars-time/src/lib.rs	0.00% <0.00%> (-100.00%)	⬇️
polars/polars-utils/src/lib.rs	0.00% <0.00%> (-100.00%)	⬇️
polars/polars-io/src/prelude.rs	0.00% <0.00%> (-100.00%)	⬇️
polars/polars-time/src/windows/test.rs	0.00% <0.00%> (-100.00%)	⬇️
polars/polars-io/src/avro/mod.rs	0.00% <0.00%> (-95.59%)	⬇️
... and 133 more

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[ritchie46]

Thanks for the quick fix!