gitignore: Add /.eggs

Quick PR to ignore the `/.eggs` folder, which appears to be generated every
time the `python setup.py test` command is run.

The content of the `./.eggs/README.txt` file:

> This directory contains eggs that were downloaded by setuptools to build,
> test, and run plug-ins.
> 
> This directory caches those eggs to prevent repeated downloads.
> 
> However, it is safe to delete this directory.

.gitignore

@@ -4,3 +4,4 @@ __pycache__
 /dist
 /yamllint.egg-info
 /build
+/.eggs