EditorConfig is a cross-editor configuration file that is pretty widely adopted: https://editorconfig.org/

Adding this to the root directory will allow editors that use the file to automatically format a few details which we already enforce with `make patchcheck`:

- always put an empty line at the end of the file;
- remove trailing whitespace;
- disallow tabs in Python (PEP 8) and C (PEP 7) files.