pyproject: Add requires-python

Together with the previous commit this allows to use uv to build the
library and run test. See
https://astral.sh/blog/uv-unified-python-packaging.

pyproject.toml

@@ -11,6 +11,7 @@ authors = [
 ]
 license = {text = "GPL-3.0"}
 keywords = ["vault", "keepass"]
+requires-python = ">=3.7"
 dependencies = [
     "pyotp>=2.9.0",
     "setuptools; python_version<'3.8'",