Fix naming-convention to flag single letter O or I variable

[ardislu]

The regex ^[lOI]$ inside should_avoid_name() will only match on the single letter variables l, O, or I, which are not recommended by the style guide.

The inner not is_upper_case_with_underscores() check is unnecessary and also allows the single letter variables O and I to pass.

Fixes #1418

[CLAassistant]

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[montyly]

great fix @ardislu. Thanks for updating the tests ;)