style: Move isort configuration to pyproject.toml

- [x] Remove flake8-import-order checks, as we only need isort for this
- [x] Clean up configuration and requirements

lib/spack/spack/cmd/style.py

@@ -219,10 +219,6 @@ def run_flake8(file_list, args):
     print_tool_header("flake8")
     flake8_cmd = which("flake8", required=True)
 
-    # Check if plugins are installed
-    if "import-order" not in flake8_cmd("--version", output=str, error=str):
-        tty.warn("style: flake8-import-order plugin is not installed, skipping")
-
     output = ""
     # run in chunks of 100 at a time to avoid line length limit
     # filename parameter in config *does not work* for this reliably