Add support for Python 3.13

[alexdewar]

This PR replaces the single use of the imghdr built-in package, which is no longer available in Python 3.13. This was the only barrier to using Python 3.13, so I've now updated the pyproject.toml and added it to the CI test matrix.

Closes #55.

[alexdewar]

Note: I opted to use the filetype package to check whether files are images or not, because it is lightweight and has no external dependencies. But if you want to do more sophisticated checking in future (e.g. to figure out whether input is XML vs HTML) then it might be worth substituting it for something like this instead.