v1.4.0

Changes

• Drop support for Python < 3.9
• Migrate to pyproject.toml and remove tox from test dependencies
• Move tests from jwt/tests to the project root
• Format code with black, isort, and pyupgrade
• Remove unused type: ignore comments and merge mypy config into pyproject.toml

Fixes

• test: Remove trailing newline from rsa_privkey.der