Fix repr error on History object

[yakky]

Fix the History object repr / str

Fix #78

[codecov]

Codecov Report

Merging #82 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #82   +/-   ##
=======================================
  Coverage   97.48%   97.48%           
=======================================
  Files           7        7           
  Lines         835      835           
  Branches       69       69           
=======================================
  Hits          814      814           
  Misses          8        8           
  Partials       13       13

Impacted Files	Coverage Δ
taiga/models/base.py	99.27% <ø> (ø)	⬆️

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[coveralls]

Coverage remained the same at 97.622% when pulling 3b0f168 on hotfix/issue-78 into e2f0de9 on master.