test: unit tests for config, filters, client and inventory building

pytest suite with mocked NetBox API responses, plus a makefile driving
venv setup, tests, linting and formatting.

pytest.ini

@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+# Copyright (C) 2026 Bryan Joshua Pedini
+[pytest]
+testpaths = tests
+addopts = -ra -q