Python 2.7 incompatibility in the iframe renderer

Plotly with python 2.7 fails when using the `iframe` renderer, due to a Python 3 call to os.makedirs with the exist_ok flag.

Minimum failing example:
`import plotly.graph_objects as go
fig = go.Figure(
    data=[go.Bar(y=[2, 1, 3])],
    layout_title_text="A Figure Displayed with the 'png' Renderer"
)
fig.show(renderer="iframe")`
