Mypy warn redundant casts

[zundertj]

Lets get this in, otherwise we may be having too many different mypy changes.

Fixes mypy "warn_redundant_casts" in #4044.

[codecov-commenter]

Codecov Report

Merging #4055 (c53bebc) into master (8406450) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4055      +/-   ##
==========================================
- Coverage   62.87%   62.85%   -0.02%     
==========================================
  Files         449      449              
  Lines       74543    74543              
==========================================
- Hits        46868    46854      -14     
- Misses      27675    27689      +14     

Impacted Files	Coverage Δ
...polars-core/src/chunked_array/iterator/par/list.rs	95.23% <0.00%> (-4.77%)	⬇️
polars/polars-arrow/src/utils.rs	74.15% <0.00%> (-1.13%)	⬇️
polars/polars-core/src/series/series_trait.rs	12.05% <0.00%> (-1.07%)	⬇️
...s/polars-core/src/series/implementations/floats.rs	59.13% <0.00%> (-0.93%)	⬇️
...lars/polars-core/src/series/implementations/mod.rs	87.17% <0.00%> (-0.77%)	⬇️
polars/polars-core/src/chunked_array/cast.rs	72.22% <0.00%> (-0.70%)	⬇️
polars/polars-time/src/series/mod.rs	56.00% <0.00%> (-0.67%)	⬇️
py-polars/polars/testing.py	93.88% <0.00%> (-0.56%)	⬇️
polars/polars-core/src/series/mod.rs	53.32% <0.00%> (-0.48%)	⬇️
...lars/polars-core/src/series/arithmetic/borrowed.rs	35.56% <0.00%> (-0.41%)	⬇️
... and 10 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8406450...c53bebc. Read the comment docs.

[stinodego]

Looks good to me!

PS: You can mark this review as "ready for review" with a button below here somewhere.