Merge pull request #13470 from typhoonzero/fix_mat_plot_req_for_py2

fix py2 matplotlib deps

Author: typhoonzero

python/requirements.txt

@@ -2,7 +2,7 @@ requests==2.9.2
 numpy>=1.12,<=1.14 #TODO:change to ">=1.12" when numpy fix bug in 1.15 and higher version
 protobuf==3.1
 recordio>=0.1.0
-matplotlib
+matplotlib==2.2.3 # TODO: let python3 paddlepaddle package use latest matplotlib
 rarfile
 scipy>=0.19.0
 Pillow