Skip doctests on Julia 1.6

[tkf]

No description provided.

[codecov]

Codecov Report

Merging #13 into master will increase coverage by 7.67%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
+ Coverage   51.54%   59.22%   +7.67%     
==========================================
  Files           1        1              
  Lines          97      103       +6     
==========================================
+ Hits           50       61      +11     
+ Misses         47       42       -5     

Flag	Coverage Δ
#unittests	59.22% <ø> (+7.67%)	⬆️

Impacted Files	Coverage Δ
src/LazyGroupBy.jl	59.22% <0.00%> (+7.67%)	⬆️

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