Pin sahi to fix object detection when installing in a fresh environment

[ethanwharris]

What does this PR do?

See discussion in #1493

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[codecov]

Codecov Report

Base: 92.35% // Head: 92.34% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (6ec4918) compared to base (34caca4).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1496      +/-   ##
==========================================
- Coverage   92.35%   92.34%   -0.01%     
==========================================
  Files         292      292              
  Lines       13160    13160              
==========================================
- Hits        12154    12153       -1     
- Misses       1006     1007       +1     

Flag	Coverage Δ
pytest	13.02% <ø> (ø)
tpu	13.02% <ø> (ø)
unittests	92.88% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/text/question_answering/model.py	93.87% <0.00%> (-0.69%)	⬇️

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