disnake.File support for io.TextIOBase files

[Enegg]

Summary

Add support for passing io.TextIOBase-d files as fp param to disnake.File.

What is the feature request for?

The core library

The Problem

At runtime aiohttp seems to handle io.TextIOBase files, so the support is sort of there.
However the parameter isn't typed as accepting them, resulting in the linter complaining.

The Ideal Solution

Allow passing io.TextIOBase directly as the fp param.

The Current Solution

# type: ignore when passing the "incompatible" type.