future dependency should be scoped to python 2 in setup.py

[kapilt]

currently there's an unconditional dep on the future library but afaics its just compatibility to python3, ideally that should be detected in the handful of places its used to avoid the shim/alias on python3. afaics there's only two uses

./aws_xray_sdk/core/plugins/ec2_plugin.py
2:from future.standard_library import install_aliases

./aws_xray_sdk/ext/sqlalchemy/util/decorators.py
4:from future.standard_library import install_aliases

[stale]

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