fix: allow IdentityDict to be assigned IdentityAccess

[tonyandrewmeyer]

Allow the access field of IdentityDict to be the enum or the string literals.

Fixes #1977

[tonyandrewmeyer]

The issue has a bunch of discussion copied from somewhere else that makes me wonder if this is too simple a fix, but it seems to solve the type: ignore at the heart of that, and the code still works since we handle the enum, and this should be backwards compatible since we're expanding not contracting. But maybe I am missing something?

[james-garner-canonical]

I think this technically runs into backwards compatibility issues with Identity.to_dict. We still convert access to a string, but the type checker doesn't know that. Previously a user could know at type checking time that identity.to_dict('access') was a str, but now the type checker will think it might be an enum member instead.