TST: test fix on GH3235 (failing because of system dtype on different systems)
BUG: GH3311 Dataframe where with a datetimelike was not correctly selecting
Merge pull request #3313 from jreback/GH3235_test

TST: test fix on GH3235 (failing because of system dtype on different sy...
Merge pull request #3312 from jreback/GH3311

BUG: GH3311 Dataframe where with a datetimelike was not correctly selecting
ENH: add df.dgrep to sandbox, add sandbox.dgrep option
TST: add tests for df.dgrep