DOCS: Update import statements in docstrings
xref https://github.com/pandas-dev/pandas/pull/21731#pullrequestreview-134460126

- [ ] ./pandas/plotting/_converter.py|134 col 17| ">>> from pandas.plotting import register_matplotlib_converters"
- [ ] pandas/plotting/_misc.py|501 col 5| >>> from pandas import read_csv
- [ ] pandas/plotting/_misc.py|502 col 5| >>> from pandas.tools.plotting import parallel_coordinates
- [x] ./pandas/util/testing.py|602 col 5| >>> from pandas.util.testing import capture_stdout https://github.com/pandas-dev/pandas/pull/42979
- [x] ./pandas/util/testing.py|650 col 5| >>> from pandas.util.testing import capture_stderr https://github.com/pandas-dev/pandas/pull/42979
- [x] ./pandas/util/testing.py|2213 col 7| >>> from pandas.util.testing import network https://github.com/pandas-dev/pandas/pull/42979
- [x] ./pandas/util/testing.py|2214 col 7| >>> from pandas.io.common import urlopen https://github.com/pandas-dev/pandas/pull/42979
- [ ] ./pandas/tseries/holiday.py|145 col 9| >>> from pandas.tseries.holiday import Holiday, nearest_workday
- [ ] ./pandas/tseries/holiday.py|146 col 9| >>> from pandas import DateOffset
- [ ] ./pandas/core/sparse/series.py|739 col 9| >>> from numpy import nan
- [ ] pandas/core/frame.py|2844 col 9| >>> from numpy.random import randn
- [ ] pandas/core/frame.py|2845 col 9| >>> from pandas import DataFrame
- [ ] ./pandas/core/algorithms.py|1519 col 5| >>> from pandas.api.extensions import take
- [ ] ./pandas/core/dtypes/concat.py|285 col 5| >>> from pandas.api.types import union_categoricals
- [ ] ./pandas/io/pytables.py|457 col 5| >>> from pandas import DataFrame
- [ ] ./pandas/io/pytables.py|458 col 5| >>> from numpy.random import randn
- [ ] ./pandas/io/json/normalize.py|138 col 5| >>> from pandas.io.json import json_normalize
- [ ] pandas/io/stata.py|2713 col 5| >>> from pandas.io.stata import StataWriter117