Convert packaging to PEP 517 with Hatchling; declare Django 6 support

[akx]

This PR:

• converts packaging from setup.py to Hatch's Hatchling.
  • For maintainers, the notable thing is that the version number is now managed in qsessions/__init__.py; otherwise, building a package should just work, as before.
  • I checked the diff between a build from main and this branch; all files are there as expected.
• adds Django 6.0 and Python 3.14 to the test/support matrix.