Add .DS_Store, *.swp, and .incremental_checker_cache.json to gitignore

[Michael0x2a]

.DS_Store appears on Macs, *.swp is a temporary file Vim creates when
editing a file, and .incremental_checker_cache.json should probably be
ignored for the same reason .mypy_cache/ should be ignored.

[gvanrossum]

Did you know you can create a personal global .gitignore? I have this in ~/.gitconfig:

[core]
    excludesfile = /Users/guido/.gitignore_global

and a series of personal excludes in ~/.gitignore_global.

Ideally a project's local .gitignore should only contain files that are specific by-products of the tools that are part of the project's build flows. In this case I think .incremental_checker_cache.json belongs in mypy's .gitignore but not .swp or .DS_Store.

There are a bunch of other lines in mypy's .gitignore that ought to be deleted (probably most, actually).