remove nltk pin

[mattseddon]

The most recent version of unstructured has moved to nltk==3.9.1. We want to use that version of unstructured now so we need to remove the pin.

[cloudflare-workers-and-pages]

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Latest commit:	9ce9553
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[mattseddon]

if this works this PR will replace #309

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.94%. Comparing base (70fe5a1) to head (9ce9553).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #332   +/-   ##
=======================================
  Coverage   86.94%   86.94%           
=======================================
  Files          90       90           
  Lines        9898     9898           
  Branches     1995     1995           
=======================================
  Hits         8606     8606           
  Misses        944      944           
  Partials      348      348           

Flag	Coverage Δ
datachain	86.87% <ø> (ø)

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