Revert "ci: replace setuptools_scm write_to with version_file"

To maintain Python 3.7 compatibility, revert the version file location reference back to
`write_to` under the `tool.setuptools_scm` section. Without doing so, the Python 3.7
test job fails with an error like the following:

```
          return cls(relative_to=name, **args)
      TypeError: __init__() got an unexpected keyword argument 'version_file'
      [end of output]

  note: This error originates from a subprocess, and is likely not a problem with pip.
error: metadata-generation-failed

× Encountered error while generating package metadata.
╰─> See above for output.
```

This reverts commit bd1ce14.

pyproject.toml

@@ -19,7 +19,7 @@ dynamic = ["version"]
 testing = ["pytest", "pytest-cov", "black", "flake8", "flake8-pyproject", "isort"]
 
 [tool.setuptools_scm]
-version_file = "si_prefix/_version.py"
+write_to = "si_prefix/_version.py"
 
 [tool.setuptools]
 packages = ["si_prefix"]