Netbox: Add supported python version

[kobayashi]

Change Type

[x] Addition
[ ] Correction
[ ] Deprecation
[ ] Cleanup (formatting, typos, etc.)

Area

[x] Installation instructions
[ ] Configuration parameters
[ ] Functionality/features
[ ] REST API
[ ] Administration/development
[ ] Other

Proposed Changes

There are no description which python versions are currently supported. Worth add the list of python version in the docs or README. I think Install Python Packges in the docs the place to add the list

[jeremystretch]

We should also consider testing. We currently test against only the minimum supported Python version (which will change to 3.6 with NetBox v2.8). We can test against multiple versions but is it going to be worth the overhead to tests on every push/merge?

[DanSheps]

I haven't looked too much into travis, but is it possible to perform certain tests only on master/develop and not each PR?

I think testing every supported version on a merge to develop would still be wise, but perhaps not on every PR.

[lampwins]

We can test against multiple versions but is it going to be worth the overhead to tests on every push/merge?

I vote yes. It really doesn't cost us anything other than maybe having to wait an extra minute or two every once in a while. I don't see this as reason _not_ to test, as it will begin to identify issues like #4220.

[kobayashi]

The place to add the description about supported version is better under Application Stack like this.

Also, I supports @lampwins's idea, so simply would add python version to the current travis.yml for testing.

python:
  - "3.5"
  - "3.6"
  - "3.7"

Any thoughts?