Cargo: add missing python package requirements

[xrmx]

When setup.py got removed they haven't been added in Cargo.toml making
the package broken at runtime.

[xrmx]

This fixes:

$ ./lel/bin/pip install celery-exporter
Collecting celery-exporter
  Using cached celery_exporter-1.5.0-cp39-cp39-manylinux1_x86_64.whl (2.2 MB)
Installing collected packages: celery-exporter
Successfully installed celery-exporter-1.5.0
$ ./lel/bin/celery-exporter 
Traceback (most recent call last):
  File "./lel/bin/celery-exporter", line 5, in <module>
    from celery_exporter.__main__ import main
  File "./lel/lib/python3.9/site-packages/celery_exporter/__main__.py", line 8, in <module>
    import click
ModuleNotFoundError: No module named 'click'