feat(python): check file target is not an existing directory

[alexander-beedie]

Closes #6181.

• Raise explicit/helpful IsADirectoryError if file target already exists and is a directory.
• Renamed format_path to normalise_filepath (now it's not just about formatting).

(As the existing format_path function was already centralised, this update covers all read/write functions that expect to be working with a file path, not just the write_csv method mentioned in the associated issue 👍)

[stinodego]

Thanks! Looks good to me 😄