Mark defunct and internal methods as deprecated

[richardm-stripe]

Summary

This PR adds deprecation annotations to classes and methods that, in my judgement, we should phase out -- at least for external users.

For things we want to keep internally but deprecate externally, I took two different approaches in different situations.

1. In some cases, I added a deprecation notice, introduced an internal (underscore-prefixed) version, and moved all internal usages to the underscore-prefixed copy.
2. In other cases, I just added the deprecation notice and added a with warnings.catch_warnings(): clause around the internal use, so that errors won't bubble down to users.

Option 2 is more conservative. Both options preserve the behavior of the function itself. However, with Option 1 could cause a behavior difference if somebody had a child class, overrode the method in question, and relied on their overloaded version to be called by the internal code in the parent class. I've used option 2 in circumstances where that was a concern.

Internal Classes

• StripeObject.ReprJSONEncoder - Used in StripeObject.__str__ for custom encoding datetimes. No external use case.

Deprecated methods

• APIResource.retrieve, - Superceded by explicit overrides on the child class.
• ListObject.retrieve, ListObject.create - odd methods that use the url from the ListObject to construct post and get requests. There is no guarantee that the API will actually support those methods, and I think it's better for users to be explicit about which API method they are calling.
• StripeObject.api_base - I don't think this has been used since 2018

Internal methods

• APIRequestor.format_app_info - internal helper used for formatting the user agent string.
• ListObject.list, SearchResultObject.search - Like ListObject.retrieve and ListObject.create -- makes a request based off of the url property. This one is used internally by pagination, but for external users it's better if they either auto-paginate or call the specific list method explicitly.
• ListObject.empty_list, SearchResultObject.empty_search_result - internal helper for pagination, no real legitimate external use.
• StripeError.construct_error_object - internal helper for deserializing errors.
• StripeObject.to_dict_recursive - internal helper for StripeObject.str
• StripeObject.stripe_id - internal helper for APIRequestor._api_encode

Not deprecated (yet)

• ListableAPIResource, CreateableAPIResource, DeleteableAPIResource, SearchableAPIResource, SingletonAPIResource, UpdateableAPIResource, @custom_method, @nested_resource_class_methods - These classes provide CRUDL methods that have all been superceded by overrides on the resources that inherit from them, but they are still in use by other parts of the library, so it's difficult to prevent DeprecationWarning from being bubbled up when users use those other, non-deprecated parts of the library that use these features.
• ListableResource.auto_paging_iter - I almost deprecated this one. Customer.auto_paging_iter(...) is just shorthand for Customer.list(...).auto_paging_iter() and feels unnecessary and non-standard, but I don't think preserving it is too harmful and it's not worth forcing users to change.