fix: typing issues, bug in infernece

[stephantul]

Small typing issues: we actually accept non-list sequences in almost all our functions, but type them as list. This should be Sequence[str]. Since str is also Sequence[str], we can just use a single type in most places, and no longer have to use a union.

Inference pipeline saving was broken (my bad)

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
model2vec/inference/model.py	66.66%	1 Missing ⚠️

Files with missing lines	Coverage Δ
model2vec/model.py	94.15% <100.00%> (+0.15%)	⬆️
model2vec/inference/model.py	92.38% <66.66%> (ø)

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