Add functools-lru-cache to setup

[ramonsaraiva]

#495

This PR dynamically adds 'backports.functools_lru_cache==1.2.1' to install_requires if python version is less than 3.

This is urgent cause arrow is broken in py2.x versions.

[codecov-io]

Codecov Report

Merging #496 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #496   +/-   ##
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  Coverage     100%   100%           
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  Files          14     14           
  Lines        3189   3189           
  Branches      229    229           
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  Hits         3189   3189

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[FRidh]

Why is a specific patch release needed? Can't it work with any 1.2? Fixing to a patch release is bad practice. Also, why not allow the latest version?