Travis is updating default Python version

[tesuji]

Copied from their changelog:

On April 16th 2019, the default Python version used to run your builds
will be updated from Python 2.7 to Python 3.6

If your builds rely on the current default Python version 2.7, you can
pin your project to continue using this version by specifying it in your
.travis.yml file:

language: python
python: '2.7'

[matprec]

cc #59838

[tesuji]

Try the triage bot again :))
@rustbot modify labels: +A-build and +A-rustbuild

[hellow554]

Maybe ping the infra team?

[jonas-schievink]

Closing in favor of #59838 (I've left a comment regarding the Travis version update)