Fix nondeterministic buildout.txt test failure

If the test happened to write to mkdir.py twice in the same second,
Python's import cache wouldn't notice that the .pyc file is stale and
would execute old code.

The 2.x docs are currently not hosted on any site :(

Add more stuff to .gitignore

README.rst: Fix link to buildout documentation

was broken when invoking setup scripts.  Fixed the test to break
properly. :/