CLN: clean up some test warnings
from a recent 3.6 build: https://travis-ci.org/pandas-dev/pandas/jobs/272847731

some of these warnings can fix / clean

```
pandas/tests/test_categorical.py::TestCategorical::()::test_tab_complete_warning
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/IPython/core/interactiveshell.py:714: UserWarning: Attempting to work in a virtualenv. If you encounter problems, please install IPython inside the virtualenv.
    warn("Attempting to work in a virtualenv. If you encounter problems, please "
pandas/tests/test_resample.py::TestResamplerGrouper::()::test_tab_complete_ipython6_warning
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/IPython/core/interactiveshell.py:714: UserWarning: Attempting to work in a virtualenv. If you encounter problems, please install IPython inside the virtualenv.
    warn("Attempting to work in a virtualenv. If you encounter problems, please "
pandas/tests/test_window.py::TestExpanding::()::test_empty_df_expanding[ls]
  source:1704: DeprecationWarning: invalid escape sequence \m
  source:3277: DeprecationWarning: invalid escape sequence \*
pandas/tests/test_window.py::TestMoments::()::test_rolling_quantile_np_percentile
  /home/travis/build/pandas-dev/pandas/pandas/core/indexes/datetimes.py:523: UserWarning: Discarding nonzero nanoseconds in conversion
    index = _generate_regular_range(start, end, periods, offset)
pandas/tests/frame/test_analytics.py::TestDataFrameAnalytics::()::test_corr_int_and_boolean
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/scipy/stats/stats.py:3577: RuntimeWarning: invalid value encountered in double_scalars
    size * (size - 1) * (size - 2))
pandas/tests/frame/test_analytics.py::TestDataFrameAnalytics::()::test_clip_mixed_numeric
  source:1895: DeprecationWarning: invalid escape sequence \d
  source:2464: DeprecationWarning: invalid escape sequence \(
pandas/tests/dtypes/test_missing.py::test_array_equivalent_compat
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/numpy/core/numeric.py:2604: FutureWarning: elementwise == comparison failed and returning scalar instead; this will raise an error or perform elementwise comparison in the future.
    return bool(asarray(a1 == a2).all())
pandas/tests/frame/test_operators.py::TestDataFrameOperators::()::test_boolean_comparison
  /home/travis/build/pandas-dev/pandas/pandas/core/internals.py:1293: DeprecationWarning: elementwise == comparison failed; this will raise an error in the future.
    result = func(values, other)
  /home/travis/build/pandas-dev/pandas/pandas/core/internals.py:1293: DeprecationWarning: elementwise == comparison failed; this will raise an error in the future.
    result = func(values, other)
pandas/tests/io/test_stata.py::TestStata::()::test_value_labels_iterator[True]
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/_pytest/python.py:143: ResourceWarning: unclosed file <_io.BufferedReader name='/tmp/tmp5x6k3vm4'>
    testfunction(**testargs)
pandas/tests/io/test_stata.py::TestStata::()::test_value_labels_iterator[False]
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/_pytest/python.py:143: ResourceWarning: unclosed file <_io.BufferedReader name='/tmp/tmpczosdkao'>
    testfunction(**testargs)
pandas/tests/io/formats/test_printing.py::TestTableSchemaRepr::()::test_publishes
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/IPython/core/interactiveshell.py:714: UserWarning: Attempting to work in a virtualenv. If you encounter problems, please install IPython inside the virtualenv.
    warn("Attempting to work in a virtualenv. If you encounter problems, please "
pandas/tests/plotting/test_frame.py::TestDataFramePlots::()::test_subplots_timeseries_y_axis_not_supported
  source:2736: DeprecationWarning: invalid escape sequence \*
pandas/tests/plotting/test_misc.py::TestDataFramePlots::()::test_parallel_coordinates_with_sorted_labels
  /home/travis/miniconda3/envs/pandas/lib/python3.6/site-packages/matplotlib/axes/_base.py:2917: UserWarning: Attempting to set identical left==right results
  in singular transformations; automatically expanding.
  left=0, right=0
    'left=%s, right=%s') % (left, right))
pandas/tests/reshape/test_pivot.py::TestPivotTable::()::test_margins_dtype_len
  source:1895: DeprecationWarning: invalid escape sequence \d
  source:2464: DeprecationWarning: invalid escape sequence \(
pandas/tests/reshape/test_pivot.py::TestPivotTable::()::test_categorical_margins
  source:1895: DeprecationWarning: invalid escape sequence \d
  source:2464: DeprecationWarning: invalid escape sequence \(
pandas/tests/reshape/test_pivot.py::TestPivotTable::()::test_categorical_margins_category
  source:1895: DeprecationWarning: invalid escape sequence \d
  source:2464: DeprecationWarning: invalid escape sequence \(
pandas/tests/reshape/test_reshape.py::TestGetDummiesSparse::()::test_include_na
  source:854: DeprecationWarning: invalid escape sequence \d
pandas/tests/util/test_testing.py::TestAssertAlmostEqual::()::test_assert_almost_equal_edge_case_ndarrays
  /home/travis/build/pandas-dev/pandas/pandas/core/dtypes/missing.py:316: DeprecationWarning: elementwise == comparison failed; this will raise an error in the future.
    return ((left == right) | (isna(left) & isna(right))).all()
  /home/travis/build/pandas-dev/pandas/pandas/core/dtypes/missing.py:316: DeprecationWarning: elementwise == comparison failed; this will raise an error in the future.
    return ((left == right) | (isna(left) & isna(right))).all()
```