djstripe.exceptions.InvalidStripeAPIKey: Invalid API key: '' when attaching a payment method

[dbartenstein]

Describe the bug
As described here https://dj-stripe.dev/dj-stripe/2.7/api_keys/, we got rid of STRIPE_LIVE_SECRET_KEY and STRIPE_TEST_SECRET_KEY.

However when calling Customer.add_payment_method, PaymentMethod.attach is called without explicit api_key param.

Fallback is djstripe_settings.STRIPE_SECRET_KEY, which will be "" in our case:

dj-stripe/djstripe/models/payment_methods.py

Line 1168 in dbe3602

api_key: str = djstripe_settings.STRIPE_SECRET_KEY,

A clear and concise description of what the bug is.

To Reproduce

*** djstripe.exceptions.InvalidStripeAPIKey: Invalid API key: ''

Software versions

• Dj-Stripe version: 2.8.3
• Python version: 3.11.8
• Django version: 4.2.11
• Stripe API version: 2020-08-27
• Database type and version: PostgreSQL 15

[dbartenstein]

Possible fix:
payment_method = PaymentMethod.attach(payment_method, stripe_customer, api_key=self.default_api_key)

dj-stripe/djstripe/models/core.py

Line 1164 in dbe3602

payment_method = PaymentMethod.attach(payment_method, stripe_customer)