Lint against python 3.9 only

[freddyaboulton]

Pull Request Description

According to a maintainer of black, we should lint against the latest version of python we support, which is 3.9 (comment). I also removed the 3.7, 3.8 elements from the lint matrix because the version of python that black checks against is determined by the values of the -t parameter not the version of the python runtime.

The main difference in lint now is just putting a comma after **kwargs hehe

---

After creating the pull request: in order to pass the release_notes_updated check you will need to update the "Future Release" section of docs/source/release_notes.rst to include this pull request by adding :pr:123.

[codecov]

Codecov Report

Merging #2586 (6f220c1) into main (f631056) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main   #2586   +/-   ##
=====================================
  Coverage   99.9%   99.9%           
=====================================
  Files        295     295           
  Lines      27055   27055           
=====================================
  Hits       27012   27012           
  Misses        43      43           

Impacted Files	Coverage Δ
...ents/estimators/classifiers/catboost_classifier.py	100.0% <ø> (ø)
...estimators/classifiers/decision_tree_classifier.py	100.0% <ø> (ø)
...ts/estimators/classifiers/elasticnet_classifier.py	100.0% <ø> (ø)
...components/estimators/classifiers/et_classifier.py	100.0% <ø> (ø)
...ts/estimators/classifiers/kneighbors_classifier.py	100.0% <ø> (ø)
...ents/estimators/classifiers/lightgbm_classifier.py	100.0% <ø> (ø)
...tors/classifiers/logistic_regression_classifier.py	100.0% <ø> (ø)
...omponents/estimators/classifiers/svm_classifier.py	100.0% <ø> (ø)
...nents/estimators/classifiers/xgboost_classifier.py	100.0% <ø> (ø)
...valml/pipelines/components/estimators/estimator.py	100.0% <ø> (ø)
... and 13 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 f631056...6f220c1. Read the comment docs.

[jeremyliweishih]

nice!

[bchen1116]

LGTM!