Add flake8 extension: comprehensions

[stinodego]

Relates to #4044

Changes:

• Added the flake8-comprehensions extension
• Fixed linting errors

[codecov-commenter]

Codecov Report

Merging #4200 (256f344) into master (3e665fd) will decrease coverage by 14.64%.
The diff coverage is 93.96%.

@@             Coverage Diff             @@
##           master    #4200       +/-   ##
===========================================
- Coverage   78.76%   64.11%   -14.65%     
===========================================
  Files         458      457        -1     
  Lines       75785    75627      -158     
===========================================
- Hits        59691    48491    -11200     
- Misses      16094    27136    +11042     

Impacted Files	Coverage Δ
polars/polars-core/src/frame/mod.rs	62.90% <25.00%> (-14.49%)	⬇️
polars/polars-io/src/csv/parser.rs	82.75% <57.14%> (-5.16%)	⬇️
polars/polars-core/src/frame/cross_join.rs	63.09% <65.85%> (-27.82%)	⬇️
py-polars/polars/io.py	73.93% <90.00%> (+1.11%)	⬆️
polars/polars-io/src/csv/read_impl.rs	79.96% <94.73%> (-0.04%)	⬇️
...olars/polars-core/src/chunked_array/ops/explode.rs	59.88% <100.00%> (-31.72%)	⬇️
polars/polars-core/src/frame/groupby/proxy.rs	59.47% <100.00%> (-7.19%)	⬇️
polars/polars-core/src/frame/hash_join/mod.rs	41.76% <100.00%> (-47.65%)	⬇️
polars/polars-core/src/utils/mod.rs	82.37% <100.00%> (+20.94%)	⬆️
polars/polars-io/src/csv/read.rs	89.42% <100.00%> (-2.89%)	⬇️
... and 221 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 0c649c3...256f344. Read the comment docs.