TST: Unable to locate core schemas in devdeps job

[pllim]

• Undo TST: Ignore asdf warning for devdeps #272
• Undo TST: Ignore asdf warnings in reproject tests #285

I added a test against dev astropy in #267 and saw these errors. Not sure how to fix. Example log: https://github.com/astropy/reproject/runs/4798998115?check_suite_focus=true

___________________ test_reproject_adaptive_roundtrip[asdf] ____________________

group = 'asdf.resource_mappings'
proxy_class = <class 'asdf.resource.ResourceMappingProxy'>

(very long traceback)

E       asdf.exceptions.AsdfWarning: asdf.extensions plugin from package asdf-astropy==0.1.3.dev4+ge779418 failed to load:
E       
E       AsdfWarning: asdf.resource_mappings plugin from package asdf==2.8.4.dev14+gc1f6cf9 failed to load:
E       
E       RuntimeError: Unable to locate core schemas

________________________ test_reproject_roundtrip[asdf] ________________________
(same as above)