fix: update license field to PEP-621 format in pyproject.toml

[pintaste]

The license field in pyproject.toml uses the deprecated string format. PEP 621 requires the table format:

# Before
license = "Apache-2.0"

# After  
license = {text = "Apache-2.0"}

This fixes build warnings on FreeBSD and other tooling that validates PEP-621 compliance.

Fixes #1444