Make tests pass after 2023-03-30

[bmwiedemann]

drop expiry on gpg keys that are used in tests

This PR was done while working on reproducible builds for openSUSE.

Background:
As part of my work on reproducible builds for openSUSE, I check that software still gives identical build results in the future.
The usual offset is +15 years, because that is how long I expect some software will be used in some places.
This showed up failing tests in our python-dulwich package build.
See https://reproducible-builds.org/ for why this matters.

[codecov]

Codecov Report

Merging #885 (f827178) into master (5559440) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #885   +/-   ##
=======================================
  Coverage   84.63%   84.63%           
=======================================
  Files          91       91           
  Lines       22348    22348           
  Branches     2403     2403           
=======================================
  Hits        18914    18914           
  Misses       3010     3010           
  Partials      424      424           

Impacted Files	Coverage Δ
dulwich/tests/test_porcelain.py	99.76% <ø> (ø)

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