DM-55493: Add safir to square_pypi_package dev dependencies

[jonathansick]

Summary

The data fixture added to both templates in 9f67f2b imports
safir.testing.data.Data in the generated tests/conftest.py, but
square_pypi_package's dev extra lists only coverage, pytest,
pytest-asyncio, mypy, documenteer, and scriv — no safir.

A freshly generated package therefore fails at test collection:

ImportError while loading conftest '.../tests/conftest.py'.
ModuleNotFoundError: No module named 'safir'

fastapi_safir_app is unaffected, since safir is a runtime dependency there.

This adds safir>=14.2.0 to the dev extra — 14.2.0 is the release that
introduced safir.testing.data.

Alternative worth considering

The minimal fix preserves the intent of 9f67f2b, but it does mean every
generated pure-Python package pulls safir into its dev environment purely
for the test-data fixture. If that isn't wanted for a generic PyPI package
template, the other option is to drop the fixture from
square_pypi_package instead and keep it in fastapi_safir_app only.
Happy to switch this PR to that approach — I picked the one that keeps
the fixture working.

Validation

• scons re-rendered the example; the regenerated
  square_pypi_package/example/pyproject.toml is included.
• templatekit check (with the two technote.toml ignores CI uses)
  passes: ✅ Passed!

References

Found while scanning template changes for the lsst-sqre maintenance
catalog. Ticket: DM-55493.

[rra]

Oh, this is a good point, we should probably talk about this in the context of our still unresolved question about the role of Safir as a library. I was going to bring that up next week when I'm back at work, since I also want to get the UWS pattern moved onto service discovery, which will require resolving that. I completely forgot that square_pypi_package didn't already depend on Safir.