Log Message:
py-braintree: updated to 4.21.0

4.21.0

Add evidence_submittable to Dispute
Add merchant_token_identifier and source_card_last4 to ApplePayCard
Add a check for empty month and year before generating expiration_date in:
CrediCard
AndroidPayCard
ApplePayCard
SamsungPayCard
VisaCheckoutCard
Add retry_ids and retried_transaction_id to Transaction object