chore: fsspec is now a strict (required) dependency

[jpivarski]

As per https://github.com/scikit-hep/awkward/wiki#api-breaking-changes-after-20 for 2.6.0, we will now require fsspec.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7f2f219) 81.89% compared to head (857b0a5) 81.90%.

Additional details and impacted files

Files	Coverage Δ
src/awkward/_connect/pyarrow.py	90.82% <ø> (+0.25%)	⬆️
src/awkward/operations/ak_from_json.py	94.03% <100.00%> (+0.45%)	⬆️
src/awkward/operations/ak_from_parquet.py	91.04% <100.00%> (-0.07%)	⬇️
src/awkward/operations/ak_to_json.py	86.25% <100.00%> (+1.25%)	⬆️
src/awkward/operations/ak_to_parquet.py	62.01% <100.00%> (ø)