Add BytesIO and StringIO typehints to File class

[Faholan]

Using a BytesIO or StringIO instance as a buffer to pass to a discord File is useful and perfectly correct.

I thus think that it would be fine to add it as a typehint.

However, I did not found a generic typehint for those two, if one exists it will of course be much better to add that one.

[ThatXliner]

Wouldn't typing.IO work?

[Faholan]

Wouldn't typing.IO work?

I checked, and neither BytesIO nor StringIO are subclasses of typing.IO

[ThatXliner]

But it's type hints

[bryanforbes]

There's a few issues at play here:

• The documentation states that if you're going to pass a file-like object, it should be opened in binary mode. I'm not sure if StringIO should work, even if it does.
• The PR should probably change BinaryIO to io.BufferedIOBase, but even then it's not quite right since that's not a protocol and file-like objects won't match.
• The io type hints don't define any protocols for these things, which means that it's not going to check the shape of the thing passed in, but the class inheritance if we use BytesIO and StringIO. I almost think we should make a protocol for this.