corporate: Store the Stripe API version.

corporate/lib/stripe.py

@@ -51,6 +51,9 @@ MIN_INVOICED_LICENSES = 30
 MAX_INVOICED_LICENSES = 1000
 DEFAULT_INVOICE_DAYS_UNTIL_DUE = 30
 
+# The version of Stripe API the billing system supports.
+STRIPE_API_VERSION = "2020-08-27"
+
 
 def get_latest_seat_count(realm: Realm) -> int:
     non_guests = (

docs/subsystems/billing.md

@@ -6,6 +6,10 @@ system requires a little bit of setup.
 To set up the development environment to work on the billing code:
 
 - Create a Stripe account
+- Ensure that the [API version](https://stripe.com/docs/api/versioning) of
+  your Stripe account is same as `STRIPE_API_VERSION` defined in
+  `corporate/lib/stripe.py`. You can upgrade to a higher version from
+  the Stripe dashboard.
 - Go to <https://dashboard.stripe.com/account/apikeys>, and add the
   publishable key and secret key as `stripe_publishable_key` and
   `stripe_secret_key` to `zproject/dev-secrets.conf`.
@@ -21,9 +25,10 @@ our code is:
 - Upgrade the API version.
 - Run `tools/test-backend --generate-stripe-fixtures`
 - Fix any failing tests, and manually look through `git diff` to understand
-  the changes.
-- If there are no material changes, commit the diff, and open a PR.
-- Ask Rishi or Tim to go to <https://dashboard.stripe.com/developers> in the
+  the changes. Ensure that there are no material changes.
+- Update the value of `STRIPE_API_VERSION` in `corporate/lib/stripe.py`.
+- Commit the diff, and open a PR.
+- Ask to Tim to go to <https://dashboard.stripe.com/developers> in the
   zulipchat Stripe account, and upgrade the API version there.
 
 We currently aren't set up to do version upgrades where there are breaking