Shippo-legacy

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**Behavioural Changes**

- Drop support for Python < 3.5
- Drop `async` and `sync` parameters in favour of `asynchronous`
- Move configurations to `shippo.config`

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**Behavioural Changes**

- Remove unnecessary email fields and amend parcel fields to match the new version

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**Behavioural Changes**

- The client now only supports the most recent version of the Shippo API (2017-03-29). For more information on versioning and upgrading your version, please refer to https://goshippo.com/docs/versioning

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**Improvements**

- SSL cert verification is now enabled by default

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**New features**

- Added support for Track and Batch Objects

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**Bugfixes**

- Change the API version header name