Relax a too-eager assertion of sourcestamp properties

[p12tic]

Fixes #5165.

[codecov]

Codecov Report

Merging #5827 (4392d9e) into master (6984bd2) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5827      +/-   ##
==========================================
- Coverage   91.89%   91.86%   -0.03%     
==========================================
  Files         337      337              
  Lines       36054    36054              
==========================================
- Hits        33131    33121      -10     
- Misses       2923     2933      +10     

Impacted Files	Coverage Δ
master/buildbot/process/buildrequest.py	95.78% <100.00%> (ø)
master/buildbot/scripts/cleanupdb.py	86.79% <0.00%> (-13.21%)	⬇️
master/buildbot/__init__.py	90.76% <0.00%> (-1.54%)	⬇️
worker/buildbot_worker/__init__.py	91.04% <0.00%> (-1.50%)	⬇️
master/buildbot/db/connector.py	98.83% <0.00%> (-1.17%)	⬇️

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