python 3.x ImportError: No module named 'cStringIO'

I had the same issue because my file was called email.py. I renamed the file and the issue disappeared.

From Python 3.0 changelog:

The StringIO and cStringIO modules are gone. Instead, import the io module and use io.StringIO or io.BytesIO for text and data respectively.

From the Python 3 email documentation it can be seen that io.StringIO should be used instead:

from io import StringIO
from email.generator import Generator

fp = StringIO()
g = Generator(fp, mangle_from_=True, maxheaderlen=60)
g.flatten(msg)
text = fp.getvalue()