keys: list fingerprints exactly as python-gnupg does it

[tigarmo]

This commit updates the logic of get_key_fingerprints() to be the same as python-gnupg's (used before 1.0):

• Don't list the fingerprints of sub-keys;
• List the fingerprints of primary keys in the order that they are listed by gpg, even if the key is expired.

The commit is the first one on the road to fixing a regression in the 1.* release series, of repository keys with more than one key (primary or sub-key) not being supported.

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• Have you signed the CLA?
• Have you successfully run tox?

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[codecov]

Codecov Report

Merging #84 (ca6b641) into hotfix/1.1.1 (1c354f2) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@               Coverage Diff                @@
##           hotfix/1.1.1      #84      +/-   ##
================================================
+ Coverage         90.44%   90.51%   +0.07%     
================================================
  Files                13       13              
  Lines               659      664       +5     
  Branches            131      133       +2     
================================================
+ Hits                596      601       +5     
  Misses               47       47              
  Partials             16       16              

Impacted Files	Coverage Δ
craft_archives/repo/apt_key_manager.py	98.36% <100.00%> (+0.07%)	⬆️

[lengau]

Nice. Thanks!