Ask pylint to target Python2.7

This isn't perfect, but it's better than not having it.  We should
ideally build with the lowest supported Python, but this makes it
easier to run pylint during development without switching venvs.

pyproject.toml

@@ -60,6 +60,7 @@ version = {attr = "vobject.__version__"}
 [tool.pylint.format]
 max-line-length = "88"
 disable = ["C0103", "C0301"]
+py-version = "2.7"
 
 [tool.isort]
 profile = "black"