[conf] Allow trailing commas in ini configuration of multiline values

[sobolevn]

Now these two samples are identical:

[mypy]
enable_error_code =
  truthy-bool,
  redundant-expr,
  unused-awaitable,
  ignore-without-code

and

[mypy]
enable_error_code =
  truthy-bool,
  redundant-expr,
  unused-awaitable,
  ignore-without-code,

I've covered some of the changed values, but not all (they are identical - no need to create so many slow tests).

I've also checked pyproject.toml. It does not have this problem: it uses [] to create arrays, so no trailing commas are used there.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[hauntsaninja]

Lgtm!