Combine used_attrs and used_names to avoid false positives

[jingw]

Description

This fixes false positives when assigning to x.some_name but reading via some_name. Note this produces false negatives in the opposite situation when x.some_name and some_name are different, so this is a trade-off in the direction of more precision / less sensitivity.

This direction is in line with pyflakes design principles: https://github.com/PyCQA/pyflakes#design-principles

it will try very, very hard to never emit false positives.

Fixing this more accurately probably requires a lot more work to trace references and analyze context.

Related Issue

221

Checklist:

• [x] My code follows the code style of this project.
• N/A I have updated the documentation in the README.md file.
• [x] I have added an entry in CHANGELOG.md.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[jendrikseipp]

Thanks!