Add #[\ReturnTypeWillChange] where needed

[Slamdunk]

See box/spout#873

[codecov]

Codecov Report

Merging #24 (6f39704) into main (152b978) will not change coverage.
The diff coverage is 97.05%.

@@            Coverage Diff            @@
##               main      #24   +/-   ##
=========================================
  Coverage     96.05%   96.05%           
  Complexity     1274     1274           
=========================================
  Files           106      106           
  Lines          3426     3426           
=========================================
  Hits           3291     3291           
  Misses          135      135           

Impacted Files	Coverage Δ
src/Reader/ODS/RowIterator.php	95.50% <83.33%> (ø)
src/Reader/CSV/RowIterator.php	100.00% <100.00%> (ø)
src/Reader/CSV/SheetIterator.php	100.00% <100.00%> (ø)
src/Reader/ODS/SheetIterator.php	100.00% <100.00%> (ø)
src/Reader/XLSX/RowIterator.php	98.11% <100.00%> (ø)
src/Reader/XLSX/SheetIterator.php	100.00% <100.00%> (ø)
... and 2 more

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