Braintree

* Add VenmoAccount
* Add support for Set Transaction Context supplementary data.

* Add transaction to subscription successfully charged webhook
* Add new ProcessorDoesNotSupportAuths error
* Add support for partial settlement transactions
* Add constants for dispute kind
* Preserve backtrace when not wrapping HTTP exceptions
* Add date_opened and date_won to dispute webhooks
* Add support for searching transactions from oauth app
* Support AMEX express checkout

* add source\_description to android pay and apple pay
* add new android pay test nonces
* add support for amex rewards transactions
* add billing\_agreement\_id to paypalaccount

* Add new test payment method nonces
* Allow passing description on PayPal transactions

* Fix oauth authentication
* Fix python 3 syntax

* Add oauth support