It seems that a test fails with numpy > 1.13.3
I'll investigate later

Here is the culprit:

>>> np.version.version
'1.13.3'
>>> a = np.float64('1.1111111111111')
>>> str(a)
'1.11111111111'

>>> np.version.version
'1.16.2'
>>> a = np.float64('1.1111111111111')
>>> str(a)
'1.1111111111111'

I really do not know how to put python 3.7 into travis matrix.
An alternative would be to use xenial and sudo:true for all builds. What do you think?

An alternative would be to use xenial and sudo:true for all builds.

We should avoid doing that. Container builds are scheduled faster (I think) and going back to VMs would be a step backwards. (they may even be discontinued at some point?)

However since you can only enable it for the 3.7 jobs it will do fine for now.

Should someone find a way to do without, I'll gladly merge.

Thank you for the PR!

This time you were too fast for me 😉 I pushed the doc and a small fix in #693.
(To your defense I was fiddling with the WIP status of the PR, I felt that travis was not starting fast enough)