Require Python 3.7 or later.

The version of Setuptools that has pyproject.toml/ PEP 621 support
requires Python 3.7 or later (supported version of Python).

pyproject.toml

@@ -32,6 +32,7 @@ keywords = [
 license = {file = "LICENSE.txt"}
 name = "sampleproject"
 readme = "README.rst"
+requires-python = ">=3.7"
 
 [project.scripts]
 sampleproject = "sampleproject:main"