HKDF extract public API

[reaperhulk]

cryptography is refactoring some of our HKDF internals and we realized you're using a private API. We've promoted extract to public in v47. This isn't released yet, but we'll be removing that old private name in 49.0.0 so I wanted to get a PR over to you. refs pyca/cryptography#13658

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.88%. Comparing base (4296eee) to head (5f96343).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/layers/tls/crypto/hkdf.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4861      +/-   ##
==========================================
- Coverage   80.88%   80.88%   -0.01%     
==========================================
  Files         368      368              
  Lines       90335    90337       +2     
==========================================
- Hits        73069    73068       -1     
- Misses      17266    17269       +3     

Files with missing lines	Coverage Δ
scapy/layers/tls/crypto/hkdf.py	98.11% <75.00%> (-1.89%)	⬇️

... and 10 files with indirect coverage changes

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[guedou]

Thanks for your PR!