TST: update tests\plotting\test_frame.py message check for mpl 3.1.0

[simonjayhawkins]

new error message for df.plot.line

https://travis-ci.org/pandas-dev/pandas/jobs/539282780

[simonjayhawkins]

TestDataFramePlots.test_subplots is also failing not part of this PR

[codecov]

Codecov Report

Merging #26577 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #26577      +/-   ##
==========================================
- Coverage   91.84%   91.83%   -0.01%     
==========================================
  Files         174      174              
  Lines       50643    50644       +1     
==========================================
- Hits        46515    46511       -4     
- Misses       4128     4133       +5

Flag	Coverage Δ
#multiple	90.37% <100%> (ø)	⬆️
#single	41.69% <100%> (-0.08%)	⬇️

Impacted Files	Coverage Δ
pandas/plotting/_compat.py	84.61% <100%> (+1.28%)	⬆️
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/frame.py	97% <0%> (-0.12%)	⬇️
pandas/util/testing.py	90.81% <0%> (-0.11%)	⬇️

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[TomAugspurger]

Thanks @simonjayhawkins