Add CLI and pyproject file

[jakubvalenta]

Motivation

Provide a convenient CLI to convert a FIT file to GPX.

Resolves: #6

Changes

• Add a pyproject file (a modern alternative to setup.py), so that this package can be installed using pip. I tried to copy the contents from the setup.py found in this archive: https://codeload.github.com/dodo-saba/fit2gpx/tar.gz/refs/tags/v0.0.7
• Configures the pip package to expose the cli() function as a fit2gpx command.
• Make fit_to_dataframes() and fit_to_gpx() work with not only file paths but also file descriptors, so that the CLI can operate on stdin and stdout.
• Sort imports a clean up whitespace.

Testing

pip install --user --upgrade .
fit2gpx 3323369944.fit 3323369944.gpx

[vortigont]

Any chance to get this merged? Pleeease?