Update `setuptools_scm` `write_to`

[braingram]

asdf/pyproject.toml

Line 82 in 7ae2d7e

write_to = "asdf/_version.py"

this is deprecated and should be updated:

https://setuptools-scm.readthedocs.io/en/latest/config/#setuptools_scm.Configuration

xref: https://jira.stsci.edu/browse/SCSB-138

Thanks @zacharyburnett for finding this!