include unistd.h in _chunker.c

[ThomasWaldmann]

With Python 3.13, Python.h no longer includes the <unistd.h> standard header file: https://docs.python.org/3.13/whatsnew/3.13.html#id8

(backport from master)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (70eed5e) 82.60% compared to head (13c6d1b) 82.24%.
Report is 3 commits behind head on 1.2-maint.

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Additional details and impacted files

@@              Coverage Diff              @@
##           1.2-maint    #7956      +/-   ##
=============================================
- Coverage      82.60%   82.24%   -0.37%     
=============================================
  Files             38       38              
  Lines          10781    10781              
  Branches        2064     2064              
=============================================
- Hits            8906     8867      -39     
- Misses          1329     1361      +32     
- Partials         546      553       +7     

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