Retire `blob_type`

[rwb27]

blob_type exists as a shorthand to create subclasses of Blob with the media type pre-filled. However, it doesn't play nicely with type checking/annotations, because types should not be the result of function calls.

I propose we remove blob_type and instead explain that Blob subclasses should be made by subclassing, e.g.

class PNGBlob(Blob):
    media_type: str = "image/png"

This is two lines rather than one, but it works properly with all the tools I've tried (mostly VSCode and mypy).

[julianstirling]

Agree, I think it is clearer with the two lines. Are they identical was on my list of things to ask.

[rwb27]

The classes are equivalent - but the two-line version above is statically defined, whereas the one-liner is dynamic and will not type check properly without a plugin.