## Summary
Checks that `append`, `extend` and `remove` methods are not called on
`__all__`. See [original
implementation](2a86db8271/pyi.py (L1133-L1138)).
```
$ flake8 --select Y026 crates/ruff/resources/test/fixtures/flake8_pyi/PYI056.pyi
crates/ruff/resources/test/fixtures/flake8_pyi/PYI056.pyi:3:1: Y056 Calling ".append()" on "__all__" may not be supported by all type checkers (use += instead)
crates/ruff/resources/test/fixtures/flake8_pyi/PYI056.pyi:4:1: Y056 Calling ".extend()" on "__all__" may not be supported by all type checkers (use += instead)
crates/ruff/resources/test/fixtures/flake8_pyi/PYI056.pyi:5:1: Y056 Calling ".remove()" on "__all__" may not be supported by all type checkers (use += instead)
```
```
$ ./target/debug/ruff --select PYI026 crates/ruff/resources/test/fixtures/flake8_pyi/PYI056.pyi --no-cache
crates/ruff/resources/test/fixtures/flake8_pyi/PYI056.pyi:3:1: PYI056 Calling ".append()" on "__all__" may not be supported by all type checkers (use += instead)
crates/ruff/resources/test/fixtures/flake8_pyi/PYI056.pyi:4:1: PYI056 Calling ".extend()" on "__all__" may not be supported by all type checkers (use += instead)
crates/ruff/resources/test/fixtures/flake8_pyi/PYI056.pyi:5:1: PYI056 Calling ".remove()" on "__all__" may not be supported by all type checkers (use += instead)
Found 3 errors.
```
ref #848
## Test Plan
Snapshots and manual runs of flake8.
33657d3a1c