Remove 3.9 environment marker for sktime

[freddyaboulton]

Pull Request Description

Fixes #3322

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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 #3332 (656ab21) into main (7b97147) will decrease coverage by 0.7%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3332     +/-   ##
=======================================
- Coverage   99.6%   99.0%   -0.6%     
=======================================
  Files        329     329             
  Lines      31977   31977             
=======================================
- Hits       31847   31626    -221     
- Misses       130     351    +221     

Impacted Files	Coverage Δ
evalml/tests/component_tests/test_utils.py	95.9% <100.0%> (ø)
evalml/tests/pipeline_tests/test_pipelines.py	99.8% <100.0%> (ø)
evalml/tests/utils_tests/test_dependencies.py	85.2% <100.0%> (ø)
evalml/automl/pipeline_search_plots.py	17.9% <0.0%> (-82.1%)	⬇️
...l/tests/automl_tests/test_pipeline_search_plots.py	23.3% <0.0%> (-76.7%)	⬇️
...ests/automl_tests/test_automl_search_regression.py	74.3% <0.0%> (-20.4%)	⬇️
.../automl_tests/test_automl_search_classification.py	83.8% <0.0%> (-12.6%)	⬇️
evalml/tests/automl_tests/test_automl_utils.py	91.1% <0.0%> (-8.9%)	⬇️
...lml/tests/automl_tests/test_iterative_algorithm.py	92.3% <0.0%> (-7.7%)	⬇️
evalml/automl/utils.py	98.5% <0.0%> (-1.5%)	⬇️
... and 2 more

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[jeremyliweishih]

LGTM

[angela97lin]

LGTM, thank you @freddyaboulton! Really cool that we can clean things up 🧹

[angela97lin]

Mini typo 😛

[freddyaboulton]

Fixed!

[angela97lin]

Should we just get rid of this initialization altogether?

[freddyaboulton]

Let's keep it in case we need to add to it at a later date but I am adding a comment now.

[angela97lin]

SGTM! Hopefully enough time has passed that most packages have added 3.9 support but it's a good structure to keep as a pattern for what we can do in these exception cases :)