ExtendedSigningKey from hd wallet: use key type from class

[gszabo]

I noticed this small discrepancy when I tried to derive the staking key from a HD wallet and export it to JSON.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.79%. Comparing base (23cccaa) to head (035b0f3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #400      +/-   ##
==========================================
+ Coverage   85.62%   85.79%   +0.16%     
==========================================
  Files          32       32              
  Lines        4202     4202              
  Branches     1052     1052              
==========================================
+ Hits         3598     3605       +7     
+ Misses        424      421       -3     
+ Partials      180      176       -4     

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[cffls]

LGTM, thanks!