Improve key validation for API_TOKEN_PEPPERS configuration

[LeoColomb]

Fixes: netbox-community/netbox-chart#1021

YAML is a loose format, and its dictionary key type is loose too.
In a situation where the configuration is processed by an external tool, an integer might be interpreted as a string.
When passing to NetBox, this mismatch raises the ImproperlyConfigured exception.
That being said, even the message printed is confusing (1 being an integer):

django.core.exceptions.ImproperlyConfigured: Invalid API_TOKEN_PEPPERS key: 1. All keys must be integers.

This change keeps the requirements while accepting converted types.