0.18.0

• Drop Python < 3.8 and Django < 2.2 support. #99
• Add backend for using default static storage ("maintenance_mode.backends.StaticStorageBackend"). (by @matmair in #97)
• Replace str.format with f-strings.
• Replace setup.py test in favor of runtests.py.
• Bump requirements, Github actions and pre-commit hooks.