title: Prevent pandas from reading None as Nan
body: <p>I have cleaned a dataset and had to replace a lot of <code>NaN</code> values with <code>None</code>. After that I saved it to a new csv file, when I read the cleaned dataset back using <code>pandas.read_csv</code>, all the <code>None</code> values are represented as <code>NaN</code>, how can I avoid this?</p>