scripts/scenarios: add requires-python to packse lock test template

packse has the ability to specify a project wide Requires-Python
constraint, but our lock template wasn't forwarding this to the
corresponding pyproject.toml. This update makes that happen.

scripts/scenarios/templates/lock.mustache

@@ -45,6 +45,9 @@ fn {{module_name}}() -> Result<()> {
           '''{{requirement}}''',
           {{/root.requires}}
         ]
+        {{#root.requires_python}}
+        requires-python = "{{.}}"
+        {{/root.requires_python}}
         "###
     )?;