Make sphinx-build work correctly

[Andrea Crotti]

There is an issue running sphinx-build now, because this function in conf.py

def index_html():
    import errno
    cwd = os.getcwd()
    try:
        os.chdir('build/sphinx/html')
        os.symlink('README.html', 'index.html')
        print 'index.html -> README.html'
    except OSError as error:
        if error.errno != errno.EEXIST:
            raise
    finally:
        os.chdir(cwd)

tries to chdir to a directory which doesn't exist yet.
Fixing this process should also make the doc building on readthedocs.

[Barry Warsaw (barry)]

The branch was merged but this bug wasn't marked Fixed Released. It was included in 3.0b1.