DOC to_datetime warning about dayfirst strictness
TST: remove double call to yahoo finance
Merge pull request #4175 from hayd/datetime_dayfirst_warning

DOC to_datetime warning about dayfirst strictness
Merge pull request #4176 from cpcloud/remove-yahoo-test

TST: remove double call to yahoo finance
BUG: period idx str map correctly returns an array of str with object dtype