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.

I presume 'root directory' mean the repository directory.

I would have to think about supporting this in IDLE.  It already, I believe (or will), enforces the two whitespace rules when saving and defaults to 4-space indents for first lines of statements.  Enforcing that for repository .py and .c files would be good, but there is currently no mechanism for per-file settings.

I did not see anything in the EditorConfig guide about what happens when .editorconfig does not start with root = true and another file is found.

> I did not see anything in the EditorConfig guide about what happens when .editorconfig does not start with root = true and another file is found.

Good question. The docs do mention this but the information is scattered across the website. The key points are:

> EditorConfig files are read top to bottom and the most recent rules found take precedence. Properties from matching EditorConfig sections are applied in the order they were read, so properties in closer files take precedence.

> When opening a file, EditorConfig plugins look for a file named .editorconfig in the directory of the opened file and in every parent directory. A search for .editorconfig files will stop if the root filepath is reached or an EditorConfig file with root=true is found.

In other words, unless root=True is specified, the editor will keep looking for .editorconfig also in parent directories as well and composing the config from all files it found.

New changeset 058fb35b57ca8c5063d16ec818e668b3babfea65 by Serhiy Storchaka in branch 'main':
bpo-44854: Remove trailing whitespaces (GH-27689)
https://github.com/python/cpython/commit/058fb35b57ca8c5063d16ec818e668b3babfea65

New changeset c0ab59f7de1906feee21c057ad433fad924d1e38 by Łukasz Langa in branch 'main':
bpo-44854: Add .editorconfig file to help enforce `make patchcheck` (GH-27638)
https://github.com/python/cpython/commit/c0ab59f7de1906feee21c057ad433fad924d1e38

New changeset a6808c6378ccfd732285ee20319658ac29eacf4c by Miss Islington (bot) in branch '3.10':
bpo-44854: Add .editorconfig file to help enforce `make patchcheck` (GH-27638)
https://github.com/python/cpython/commit/a6808c6378ccfd732285ee20319658ac29eacf4c

New changeset 7f88aeadc19b1d3ece4723efb240e6d6753570b9 by Miss Islington (bot) in branch '3.8':
bpo-44854: Add .editorconfig file to help enforce `make patchcheck` (GH-27638) (GH-27713)
https://github.com/python/cpython/commit/7f88aeadc19b1d3ece4723efb240e6d6753570b9

New changeset d27e2f4d118e7a9909b6a3e5da06c5ff95806a85 by Miss Islington (bot) in branch '3.9':
bpo-44854: Add .editorconfig file to help enforce `make patchcheck` (GH-27638) (GH-27714)
https://github.com/python/cpython/commit/d27e2f4d118e7a9909b6a3e5da06c5ff95806a85

Thanks everybody for review! ✨ 🍰 ✨