Explicitly add fsspec requirement to fix rtd build

[ethanwharris]

What does this PR do?

RTD build is failing because it doesn't know to mock out the fsspec library as it's not explicitly given in our requirements. This fixes it.

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[codecov]

Codecov Report

Merging #1395 (afc19a3) into master (95e19d3) will decrease coverage by 7.83%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1395      +/-   ##
==========================================
- Coverage   92.90%   85.06%   -7.84%     
==========================================
  Files         285      285              
  Lines       12778    12778              
==========================================
- Hits        11871    10870    -1001     
- Misses        907     1908    +1001     

Flag	Coverage Δ
unittests	85.06% <ø> (-7.84%)	⬇️

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

Impacted Files	Coverage Δ
...lash/image/embedding/vissl/transforms/utilities.py	15.15% <0.00%> (-84.85%)	⬇️
flash/image/embedding/heads/vissl_heads.py	35.29% <0.00%> (-64.71%)	⬇️
flash/image/embedding/losses/vissl_losses.py	37.50% <0.00%> (-62.50%)	⬇️
flash/image/embedding/vissl/adapter.py	31.63% <0.00%> (-62.25%)	⬇️
flash/image/detection/input.py	38.88% <0.00%> (-61.12%)	⬇️
...age/face_detection/backbones/fastface_backbones.py	42.10% <0.00%> (-57.90%)	⬇️
flash/image/face_detection/model.py	36.84% <0.00%> (-55.27%)	⬇️
flash/image/face_detection/input_transform.py	46.87% <0.00%> (-53.13%)	⬇️
flash/image/face_detection/output_transform.py	47.05% <0.00%> (-52.95%)	⬇️
flash/image/detection/output.py	43.47% <0.00%> (-52.18%)	⬇️
... and 54 more

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