Fix warning in ast as of python 3.8

[jamescooke]

/usr/lib/python3.8/ast.py:360: PendingDeprecationWarning: visit_Str is deprecated; add visit_Constant

Currently used here:

flake8-aaa/src/flake8_aaa/helpers.py

Lines 191 to 192 in ee1dd04

	def visit_Str(self, node) -> None:
	self.add_footprint(node)