Add version file directive to pyproject

Author: Corey Ostrove

pyproject.toml

@@ -97,5 +97,6 @@ Repository = 'https://github.com/sandialabs/pyGSTi'
 Download ='https://github.com/sandialabs/pyGSTi/tarball/master'
 
 [tool.setuptools_scm]
+version_file = "pygsti/_version.py"
 version_scheme = "only-version"
 local_scheme = "no-local-version"