Fix warnings in network/website script

We get a bunch of warnings in our network/website script we should fix:

• /usr/lib/python3/dist-packages/sklearn/externals/joblib.py:1: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses import imp
• /srv/metrics.torproject.org/metrics/monitoring/network/website:31: UserWarning: Boolean Series key will be reindexed to match DataFrame index. df = df [series.node == 'relay' ] + /srv/metrics.torproject.org/metrics/monitoring/network/website:46: UserWarning: Boolean Series key will be reindexed to match DataFrame index. df = df [series.node == 'bridge' ]
• /srv/metrics.torproject.org/metrics/monitoring/network/website:41: RuntimeWarning: invalid value encountered in double_scalars value = (trend[len(trend)-1]-trend[0]) / trend[0] * 100