Feature: accept any buffer input in the file preparation APIs

[ncoghlan]

(Note: this covers both the file and image preparation APIs)

The various prepare_file and prepare_image methods and functions currently accept the following input types:

• str (filesystem path)
• os.PathLike[str] (filesystem path)
• bytes (raw file content)
• BinaryIO (raw file content)

Rather than only accepting bytes, the file and image preparation APIs should use typing_extensions.Buffer to indicate they accept any type implementing the buffer protocol (using memoryview to extract the raw file content from such types, replacing the current dedicated bytes handling).

[ArEnSc]

Ok I got this one.

[ArEnSc]

#148