Add undocumented but used by 3rdparties Transport._sendfile_compatibl…

[tarasko]

…e attribute

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.80%. Comparing base (ed69d71) to head (71230c5).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
- Coverage   89.98%   89.80%   -0.19%     
==========================================
  Files          10       10              
  Lines        2217     2217              
  Branches       24       24              
==========================================
- Hits         1995     1991       -4     
- Misses        211      215       +4     
  Partials       11       11              

Flag	Coverage Δ
Linux	85.38% <100.00%> (ø)
Windows	82.13% <66.66%> (+0.04%)	⬆️
macOS	81.77% <100.00%> (-0.19%)	⬇️

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