Improve periodic update handling

[gaborbernat]

Resolves #1883.

[codecov]

Codecov Report

Merging #1892 into master will increase coverage by 0.01%.
The diff coverage is 98.18%.

@@            Coverage Diff             @@
##           master    #1892      +/-   ##
==========================================
+ Coverage   93.96%   93.98%   +0.01%     
==========================================
  Files          86       86              
  Lines        4175     4220      +45     
==========================================
+ Hits         3923     3966      +43     
- Misses        252      254       +2     

Flag	Coverage Δ
#tests	93.98% <98.18%> (+0.01%)	⬆️

Impacted Files	Coverage Δ
src/virtualenv/report.py	97.05% <75.00%> (-2.95%)	⬇️
src/virtualenv/seed/wheels/acquire.py	100.00% <100.00%> (ø)
src/virtualenv/seed/wheels/periodic_update.py	99.04% <100.00%> (+0.21%)	⬆️
src/virtualenv/seed/embed/base_embed.py	96.22% <0.00%> (-1.89%)	⬇️

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