WARN: some warnings on extension tests

[jreback]

pandas/tests/extension/decimal/test_decimal.py::TestMethods::()::test_value_counts[data-True]
  source:137: DeprecationWarning: invalid escape sequence \(
pandas/tests/extension/decimal/test_decimal.py::TestMethods::()::test_value_counts[data_missing-True]
  source:137: DeprecationWarning: invalid escape sequence \(

https://travis-ci.org/pandas-dev/pandas/jobs/357658822

[jreback]

cc @TomAugspurger