Exporting schema with type that inherits multiple interfaces outputs invalid schema

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* **What is the current behavior?**

When converting a schema object to string containing type with multiple interfaces results to invalid notation with comma as separating character:

`type Potato implements Food, Vegetable ...`

* **What is the expected behavior?**

`type Potato implements Food & Vegetable ...`

* **Please tell us about your environment:**

graphene: 2.1.8
graphene-django: 2.10.1
graphene-federation: 0.1.0

Platform: Mac

Start by tracing Graphene's schema-object-to-string export path for interface lists. Reproduce the issue with a type implementing multiple interfaces and compare the generated notation with the expected ampersand-separated form. Done means the exported schema is valid and the existing schema serialization tests cover this case.

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