STYLE disallow assigning to single-letter variable names

[MarcoGorelli]

We keep leaving this comment to each other and to contributors in review, so let's add a rule for it

PR should be made to https://github.com/pandas-dev/pandas-dev-flaker if anyone wants to give this a go

[jbrockmendel]

id like this, but in fairness its mostly me leaving that comment

[MarcoGorelli]

😆 pretty sure I've seen it from others too

@mgmanzella tagging you in case you'd be interested in giving this a go, as you'd expressed interest in implementing more linting rules

[MarcoGorelli]

done in pandas-dev-flaker

next step will be to gradually introduce it to the pandas codebase

[MarcoGorelli]

closing for now as the PR's already been made to pandas-dev-flaker, enabling it will be another issue