Update for ASDF standard 1.6.0 #856

Job	Run time
setup	4s
wheel_building / Build source and wheel distribution	5m 46s
core / Load tox environments	6s
asdf-schemas / Load tox environments	5s
asdf-schemas / asdf (ubuntu-latest)	2m 38s
asdf-schemas / asdf-standard (ubuntu-latest)	1m 26s
asdf-schemas / asdf-transform-schemas (ubuntu-latest)	1m 32s
asdf-schemas / asdf-coordinates-schemas (ubuntu-latest)	2m 46s
core / py39-parallel-cov (ubuntu-latest)	2m 24s
core / py39-test-devdeps-parallel-cov (ubuntu-latest)	2m 55s
core / py39-astropylts-parallel-cov (ubuntu-latest)	2m 20s
core / py39-transformlts-parallel-cov (ubuntu-latest)	2m 46s
dev	0s
oldest	0s
numpy	0s
test	0s
	24m 48s

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