Issue 71 fix lease util keep

[Revolution1]

fix #71

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@658478b). Click here to learn what that means.
The diff coverage is 91.42%.

@@            Coverage Diff            @@
##             master      #90   +/-   ##
=========================================
  Coverage          ?   91.51%           
=========================================
  Files             ?       52           
  Lines             ?     2933           
  Branches          ?      303           
=========================================
  Hits              ?     2684           
  Misses            ?      145           
  Partials          ?      104

Impacted Files	Coverage Δ
tests/test_lease_util.py	100% <100%> (ø)
tests/test_lease_apis.py	100% <100%> (ø)
etcd3/stateful/lease.py	84.15% <88.88%> (ø)

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