CI: move 2.7_SLOW -> 3.6_SLOW

[jreback]

No description provided.

[codecov]

Codecov Report

Merging #20559 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #20559      +/-   ##
==========================================
- Coverage   91.83%   91.82%   -0.02%     
==========================================
  Files         152      152              
  Lines       49260    49268       +8     
==========================================
- Hits        45240    45238       -2     
- Misses       4020     4030      +10

Flag	Coverage Δ
#multiple	90.2% <ø> (-0.02%)	⬇️
#single	41.89% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/plotting/_converter.py	65.07% <0%> (-1.74%)	⬇️
pandas/core/frame.py	97.19% <0%> (ø)	⬆️
pandas/core/series.py	93.88% <0%> (+0.02%)	⬆️
pandas/util/testing.py	84.73% <0%> (+0.2%)	⬆️

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