Drop support for python 2.7 ?

[pbthorste]

Python 2.7 is not supported anymore and ecs-deploy doesn't really work with it anymore.

When trying to run a deploy under python 2.7 this error appears:

Unknown parameter in input: "runtimePlatform", must be one of: 

Because the last supported version of boto3 did not have this parameter:
https://boto3.amazonaws.com/v1/documentation/api/1.17.112/reference/services/ecs.html?highlight=register_task_definition#ECS.Client.register_task_definition

[hugovk]

Please see PR #208.

[pbthorste]

Nicely done :)