Migration guide for v4

Changes in v4.0.0 - 2022-08-01

"⚠️" symbol highlights breaking changes.

Removed deprecated AlipayAccount, BitcoinReceiver, BitcoinTransaction, IssuerFraudRecord, Recipient, RecipientTransfer, and ThreeDSecure classes.
Removed deprecated Charge.update_dispute and Charge.close_dispute methods that were using legacy REST API endpoint. Prefer Dispute.modify and Dispute.close
Removed deprecated Card.details method and CardDetails resource. The REST API endpoint is not longer supported.
Removed the deprecated Source.source_transactions method. Prefer SubscriptionItem.list_source_transactions
```
# before
source.source_transactions()
# after
Source.list_source_transactions(item.id)
```
Removed the deprecated SubscriptionItem.usage_record_summaries method. Prefer SubscriptionItem.list_usage_record_summaries
```
# before
item.usage_record_summaries()
# after
SubscriptionItem.list_usage_record_summaries(item.id)
```

Removed the deprecated Charge.refund method. Prefer Refund.create

# before
charge.refund()
# after
Refund.create(charge=charge.id)

To be consistent with other resource methods, ApplicationFee.refund returns an instance of ApplicationFee and doesn't mutate the instance of ApplicationFee. To revert to old behavior:
```
# before
fee.refund()
# after
fee = fee.refund()
```
To be consistent with other resource methods, the Customer.delete_discount no longer resets the discount property to None and returns the deleted discount instead. If you were relying on this behavior, reset the discount property manually:
```
# before
customer.delete_discount()
# after
customer.delete_discount()
customer.discount = None
```
The LineItem resource now inherits from StripeObject as it has no methods of it's own.
To be consistent with other resource methods, the Subscription.delete_discount returns an instance of deleted discount and doesn't mutate the instance of Subscription. To revert to old behavior:
```
# before
subscription.delete_discount()
# after
subscription = subscription.delete_discount()
```
Update the CA certificate bundle.