add requests to install dependencies

[nlgranger]

When importing pixiedust_node in a notebook, the following error happens:

---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
<ipython-input-9-327dd5855a1c> in <module>()
      8 from scipy.interpolate import interp1d
      9 from skimage.draw import bezier_curve
---> 10 import pixiedust_node

~/dev/readonce/venv/lib/python3.6/site-packages/pixiedust_node/__init__.py in <module>()
     19 from IPython.core.error import TryNext
     20 import warnings
---> 21 from .node import Node, Npm
     22 import os
     23 from pixiedust.utils.shellAccess import ShellAccess

~/dev/readonce/venv/lib/python3.6/site-packages/pixiedust_node/node.py in <module>()
     11 import IPython
     12 import pandas
---> 13 from pixiedust.display import display
     14 from pixiedust.utils.environment import Environment
     15 from pixiedust.utils.shellAccess import ShellAccess

~/dev/readonce/venv/lib/python3.6/site-packages/pixiedust/__init__.py in <module>()
     29 
     30     #shortcut to logging
---> 31     import pixiedust.utils.pdLogging as pdLogging
     32     logger = pdLogging.getPixiedustLogger()
     33     getLogger = pdLogging.getLogger

~/dev/readonce/venv/lib/python3.6/site-packages/pixiedust/utils/__init__.py in <module>()
     16 
     17 import os
---> 18 from . import storage
     19 import pkg_resources
     20 import binascii

~/dev/readonce/venv/lib/python3.6/site-packages/pixiedust/utils/storage.py in <module>()
     26 from pkg_resources import get_distribution
     27 from re import search
---> 28 from requests import post
     29 from os import environ as env
     30 from pixiedust.utils.printEx import *

So the requests package should probably be explicitely listed in install_requires.