in python, use enum types instead of literals for type annotations

[gvilums]

Issue

The generated Python type annotations use typing.Literal with a list of all enum variants instead of using the name of the enum directly.
This makes manipulating the generated flatbuffer enum types cumbersome, as the full list of variants needs to be enumerated.

Fix

Use the name of the enum directly.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.