`catalog.roblox.com` endpoint support

[tomodachi94]

This adds support for the catalog.roblox.com endpoint. Related changes include:

• Bumping Python to 3.8 (discussion in RoAPI Discord)
• The introduction of two new classes for catalog items, CatalogItem and LimitedCatalogItem.
  • Limited items contain special attributes not present in a normal CatalogItem, so I figured a separate class would aid in maintainability.
• Modifying partials.partialuser and partials.partialgroup to include two new classes that handle user/group data from this endpoint.
• A new exception class, exceptions.CatalogItemNotFound.
• Two new functions in client.Client, get_catalog_items and get_base_catalog_items.
  • Do we want singular variants of these functions for consistency? Something like get_catalog_item and get_base_catalog_item.
• (I think that's it?)

Things left to do

• [x] Finish documentation for CatalogItem and LimitedCatalogItem
• [ ] Double-check that I imported everything in all the right spots
• [ ] Identify other potential values for catalog.CatalogItemTypes
• The following endpoints:
  • [ ] categories
  • [ ] subcategories
  • [ ] asset-to-category
  • [ ] get-topics
  • [ ] more?