build: add Python code formatting with black

NOTE: this will fail tests as of this commit, as the code is not
formatted yet.

We didn't explicitly specify a minimum Python version before that, so I
used v3.7 here, because it's the minimal version Ruff - a Python linter
I will add in the next commit - supports.

test/python-black-check.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+if [ -z "$MESON_SOURCE_ROOT" ]; then
+	echo "Expected \$MESON_SOURCE_ROOT to be set. Are you sure you are running this through ninja?"
+	exit 100
+fi
+
+cd "$MESON_SOURCE_ROOT" || exit 101
+
+black --check $(git ls-files "*.py" "*.py.in")