backwards compatibility with python 2.7

[malachany]

container-transform is currently not backwards compatible with python 2.7:

Traceback (most recent call last): File "/usr/local/bin/container-transform", line 9, in <module> load_entry_point('container-transform==1.1.5', 'console_scripts', 'container-transform')() File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources/__init__.py", line 565, in load_entry_point return get_distribution(dist).load_entry_point(group, name) File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources/__init__.py", line 2697, in load_entry_point return ep.load() File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources/__init__.py", line 2370, in load return self.resolve() File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources/__init__.py", line 2376, in resolve module = __import__(self.module_name, fromlist=['__name__'], level=0) File "/Library/Python/2.7/site-packages/container_transform/__init__.py", line 3, in <module> from .converter import Converter File "/Library/Python/2.7/site-packages/container_transform/converter.py", line 3, in <module> from .compose import ComposeTransformer File "/Library/Python/2.7/site-packages/container_transform/compose.py", line 6, in <module> from .transformer import BaseTransformer File "/Library/Python/2.7/site-packages/container_transform/transformer.py", line 38 class BaseTransformer(object, metaclass=ABCMeta): ^ SyntaxError: invalid syntax