Fixed compat with old python versions when packaging.

Author: Sylvain MARIE

setup.py

@@ -33,6 +33,11 @@
 setup(
     download_url=DOWNLOAD_URL,
     use_scm_version={
-        "write_to": "src/pytest_cases/_version.py"
+        "version_file": "src/pytest_cases/_version.py",
+        "version_file_template": """# file generated by setuptools_scm
+# don't change, don't track in version control
+__version__ = version = '{version}'
+__version_tuple__ = version_tuple = {version_tuple}
+"""
     },  # we can't put `use_scm_version` in setup.cfg yet unfortunately
 )