Ref issue:

- #1408

Changes:

- Add `.editorconfig`
- Add `pre-commit-hooks`:

    ```yaml
    - repo: https://github.com/pre-commit/pre-commit-hooks
      rev: v4.4.0
      hooks:
        - id: check-symlinks
        - id: destroyed-symlinks
        - id: trailing-whitespace
        - id: end-of-file-fixer
        - id: check-yaml
        - id: check-toml
        - id: check-ast
        - id: check-added-large-files
        - id: check-merge-conflict
        - id: check-executables-have-shebangs
        - id: check-shebang-scripts-are-executable
        - id: detect-private-key
        - id: debug-statements
    ```
- Add `flake8` to `pre-commit` config and add `.flake8` file
- Use `pre-commit` for style tests in CI
- Run `pre-commit` and fix existing violations:
    - fix trailing spaces
    - fix end-of-files
    - fix mod file mode with `chmod -x`
    - run `autopep8` on existing code
    - fix `flake8` violations