fix some pytest and sqlalchemy warnings

[wolfv]

This fixes a couple of pytest and sqlalchemy warnings.

[codecov-commenter]

Codecov Report

Merging #502 (2817bf9) into master (94a60fd) will decrease coverage by 0.89%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #502      +/-   ##
==========================================
- Coverage   83.19%   82.30%   -0.90%     
==========================================
  Files          78       78              
  Lines        5879     5882       +3     
==========================================
- Hits         4891     4841      -50     
- Misses        988     1041      +53     

Impacted Files	Coverage Δ
quetz/authentication/registry.py	85.71% <100.00%> (ø)
quetz/dao.py	91.08% <100.00%> (+0.01%)	⬆️
quetz/db_models.py	96.89% <100.00%> (ø)
quetz/jobs/models.py	92.59% <100.00%> (ø)
quetz/jobs/runner.py	90.86% <100.00%> (+0.08%)	⬆️
quetz/tasks/workers.py	77.14% <100.00%> (ø)
quetz/testing/mockups.py	100.00% <100.00%> (ø)
quetz/utils.py	85.43% <100.00%> (ø)
quetz/pkgstores.py	42.54% <0.00%> (-11.63%)	⬇️

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