BUG: generate_asdf.py should avoid running when GWCS is not installed

[pllim]

GWCS is only an optional test dependency

reproject/pyproject.toml

Line 37 in 7aaeb13

"gwcs",

so I would expect the test suite to not crash when GWCS is not installed

reproject/reproject/tests/data/generate_asdf.py

Line 5 in 7aaeb13

import gwcs.coordinate_frames as cf

but it crashes, see https://github.com/astropy/astropy-integration-testing/actions/runs/11073838092/job/30771295426

[pllim]

Still a problem during astropy v7.0 integration testing. I worked around it by forcing gwcs to install downstream.