CI: Disable allowed failures for Python 2.7 nightly tests

[effigies]

This may be the root of the Pandas nightly failures.

[codecov]

Codecov Report

Merging #315 into master will increase coverage by 1.47%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #315      +/-   ##
=========================================
+ Coverage   73.03%   74.5%   +1.47%     
=========================================
  Files          24      24              
  Lines        2551    2742     +191     
  Branches      624     711      +87     
=========================================
+ Hits         1863    2043     +180     
- Misses        506     514       +8     
- Partials      182     185       +3

Flag	Coverage Δ
#unittests	74.5% <ø> (+1.47%)	⬆️

Impacted Files	Coverage Δ
bids/layout/layout.py	81.65% <0%> (+6.18%)	⬆️
bids/config.py	83.01% <0%> (+10.51%)	⬆️

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[tyarkoni]

Wooooo!

[effigies]

I'm going to submit a PR to pandas to fix this, rather than add a dependency we don't directly use.

[effigies]

Fix should be in: pandas-dev/pandas#24170

I expect this will fail now, but can be rerun in the morning and will pass.