ImgLabeling conversions use suboptimal tmp directory

In the `Labeling <-> ImgLabeling` conversion code in `convert.py`, temporary paths of the form `"./tmp"` and `os.getcwd() + "/tmp"` are used. There are a couple of issues with this:

1. Inconsistency between those two expressions.
2. Use of forward slashes might not work as expected on Windows (testing probably needed).
3. Creation and use of temp folders from the CWD is generally considered bad practice. Better is to use dedicated temp file management functions of the language standard library. In Python this would be the [`tempfile` module](https://docs.python.org/3/library/tempfile.html).

Read the Labeling ImgLabeling conversion code in convert.py and compare its two temporary-path expressions. Consult Python's tempfile module documentation, then run the existing conversion tests and add or perform Windows-focused testing to verify temporary paths work consistently and are not created from the current working directory.

Written by the indexing model from the issue text.