limit tomli dependency to python < 3.11

[tobypeterson]

The only use of tomli is in genreqs.py:

try:
    import tomllib  # Python 3.11+
except ImportError:
    import tomli as tomllib  # fallback pour Python <3.11

However, the dependency is declared unconditionally - should add ; python_version < "3.11"

[PierreRaybaut]

Thanks for the feedback.
That is done in the forthcoming v3.13.