stripe: Upgrade stripe API to 2020-08-27 version.

This upgrades the Stripe API to the most recent version. Going through
the Git history, it looks like our current API version is at 2019-03-14.

The API version should be manually changed in Stripe dashboard at the same
time as the commit is deployed in production.

Backward incompatible changes that are relevant to our codebase between
(2019-03-14, 2020-08-27].
* 2020-08-27 - The `sources` property on Customers is no longer included by
  default.
* 2020-03-02 - Nothing applicable
* 2019-12-03 - The `id` field of all invoice line items have changed and are
  now prefixed
  with `il_`. We only rely on this while we normalize the fixtures.
* 2019-11-05 - Nothing applicable
* 2019-10-17 - The `billing` attribute on invoices, subscriptions, and
  subscription schedules is renamed to`collection_method`. The invoice
  change is the one that is relevant to us.
* The customer object’s `account_balance` value has been renamed to
  `balance`. Only used for the stubs at the moment.
* 2019-10-08 - Nothing applicable
* 2019-09-09 - Nothing applicable
* 2019-08-14 - Nothing applicable
* 2019-05-16 - Nothing applicable

https://stripe.com/docs/upgrades

Also normalize the following IDs in stripe fixtures

* price_[A-Za-z0-9]{24}
* prod_[A-Za-z0-9]{14}
* pi_[A-Za-z0-9]{24}
* il_[A-Za-z0-9]{24}

corporate/lib/stripe.py

@@ -277,7 +277,7 @@ def catch_stripe_errors(func: CallableT) -> CallableT:
 
 @catch_stripe_errors
 def stripe_get_customer(stripe_customer_id: str) -> stripe.Customer:
-    return stripe.Customer.retrieve(stripe_customer_id, expand=["default_source"])
+    return stripe.Customer.retrieve(stripe_customer_id, expand=["default_source", "sources"])
 
 
 @catch_stripe_errors
@@ -337,7 +337,7 @@ def do_replace_payment_source(
     )
     if pay_invoices:
         for stripe_invoice in stripe.Invoice.list(
-            billing="charge_automatically", customer=stripe_customer.id, status="open"
+            collection_method="charge_automatically", customer=stripe_customer.id, status="open"
         ):
             # The user will get either a receipt or a "failed payment" email, but the in-app
             # messaging could be clearer here (e.g. it could explicitly tell the user that there
@@ -642,15 +642,15 @@ def process_initial_upgrade(
         )
 
         if charge_automatically:
-            billing_method = "charge_automatically"
+            collection_method = "charge_automatically"
             days_until_due = None
         else:
-            billing_method = "send_invoice"
+            collection_method = "send_invoice"
             days_until_due = DEFAULT_INVOICE_DAYS_UNTIL_DUE
 
         stripe_invoice = stripe.Invoice.create(
             auto_advance=True,
-            billing=billing_method,
+            collection_method=collection_method,
             customer=customer.stripe_customer_id,
             days_until_due=days_until_due,
             statement_descriptor="Zulip Standard",
@@ -795,14 +795,14 @@ def invoice_plan(plan: CustomerPlan, event_time: datetime) -> None:
 
     if invoice_item_created:
         if plan.charge_automatically:
-            billing_method = "charge_automatically"
+            collection_method = "charge_automatically"
             days_until_due = None
         else:
-            billing_method = "send_invoice"
+            collection_method = "send_invoice"
             days_until_due = DEFAULT_INVOICE_DAYS_UNTIL_DUE
         stripe_invoice = stripe.Invoice.create(
             auto_advance=True,
-            billing=billing_method,
+            collection_method=collection_method,
             customer=plan.customer.stripe_customer_id,
             days_until_due=days_until_due,
             statement_descriptor="Zulip Standard",