Fix generating invalid trailing commas in import statements

[davidshepherd7]

Summary

Fixes #532

Test Plan

Unit tests added/tweaked for this case, also the remove imports codemod tests serve as an integration test.

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[zsol]

Thanks for your contribution! This all looks good to me. The logic will also need to be ported to

LibCST/native/libcst/src/nodes/statement.rs

Line 782 in 46509dd

for (i, alias) in aliases.iter().enumerate() {

before merging.

[Kludex]

Hello. Something I can do to speed up this?

[davidshepherd7]

Hello. Something I can do to speed up this?

Yeah, I don't really have time for open source work these days so if someone can figure out and implement the missing rust part that would speed things up a lot.

[sureshjoshi]

Are the Rust and Python implementations intended to be kept in lock-step?

I just started using libcst and ran into this bug, but it looks like it was "partially" solved already?