Fix CacheStamp issue with old caches

[Erotemic]

To reproduce:

docker run -it python3 bash

pip install ubelt==1.0.0
python -c "import ubelt; ubelt.CacheStamp('foo', '.').renew()"
pip install ubelt==1.1.0
python -c "import ubelt; ubelt.CacheStamp('foo', '.').expired()"

[codecov]

Codecov Report

Merging #116 (40f6248) into main (5064cc2) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #116   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           28        28           
  Lines         3207      3207           
  Branches       646       646           
=========================================
  Hits          3207      3207           

Impacted Files	Coverage Δ
ubelt/util_cache.py	100.00% <100.00%> (ø)

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