Describe the bug
The .gromov.gromov wasserstein method fails (TypeError) when the cost matrices are very similar but not the same

To Reproduce
The full code is available at
https://colab.research.google.com/drive/1IhnOqeLV51gWE8FodnBsgR5cQC_w2EkL

• How was POT installed [pip]

Sys specifications

Linux-3.10.0-327.22.2.el7.x86_64-x86_64-with-centos-7.2.1511-Core
Python 3.4.3 (default, Apr 28 2015, 11:29:27) 
[GCC 4.9.2]
NumPy 1.16.2
SciPy 1.2.1
POT 0.5.1