tox: use build module to create sdist

This avoids having to install the module to properly version the sdist
tarball.

Author: radhermit

tox.ini

@@ -107,6 +107,5 @@ commands =
 
 [testenv:sdist]
 description = build the source distribution
-base = tox
-allowlist_externals = cp
-commands = cp -a {toxworkdir}/{package_env}/dist {toxinidir}
+deps = build
+commands = python -m build --sdist