Drop python 2.7 support - add travis for python 3.7

[Froskekongen]

Updated #499. Here, I also update python versions to 3.6 and 3.7.

[Froskekongen]

@MaxBenChrist: I continued the work in #499 to remove python 2.7 support. I tried adding pandas 0.25 at the same time, but two tests fail, so it should be fixed in a subsequent PR.

[Froskekongen]

@MaxBenChrist: Any reason for holding back this PR?

[tomzx]

I'd also like to see this PR being merged so that we can move forward and support pandas=>0.25.

@Froskekongen It's already a big PR, but it would be nice if it also included relaxing the constraint on pandas, at best by adding !=0.24.0, although I wouldn't mind not supporting old versions of pandas (<=0.25.0).

[nils-braun]

So, first of all thanks to @Froskekongen and @hugovk for this PR(s). I (also) think it is a good idea to drop 2.7 support.
I think we can and should separate the two parts - pandas and python version - if possible so it is easier to revert if needed.
I am adding some review comments, but most of them are minor things.

[nils-braun]

As I said, most of them are minor. I can also implement some of them if you want.

[coveralls]

Coverage remained the same at 97.449% when pulling 04363d2 on Froskekongen:p27_fix into 11e5c07 on blue-yonder:master.

[nils-braun]

Nice Nice Nice.
We finally get rid of it :-)