feat: Add python3.9 support

Codecov Report

Attention: Patch coverage is 70.27027% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
model2vec/distill/tokenizer.py	56.00%	11 Missing :warning:

Files with missing lines	Coverage Δ
model2vec/distill/distillation.py	`98.13% <100.00%> (+0.01%)`	:arrow_up:
model2vec/distill/inference.py	`95.16% <100.00%> (+0.07%)`	:arrow_up:
model2vec/distill/utils.py	`100.00% <100.00%> (ø)`
model2vec/hf_utils.py	`62.22% <100.00%> (+0.42%)`	:arrow_up:
model2vec/model.py	`96.66% <100.00%> (ø)`
tests/test_distillation.py	`92.85% <100.00%> (+0.10%)`	:arrow_up:
tests/test_utils.py	`100.00% <100.00%> (ø)`
model2vec/distill/tokenizer.py	`72.72% <56.00%> (+1.54%)`	:arrow_up: