Changed default zoneinfo file format to gz.
Changed default zoneinfo file format to gz.

jenshnielsen/dateutil,sprymix/python-dateutil,sprymix/dateutil,abalkin/dateutil,pganssle/dateutil-test-codecov,pganssle/dateutil-test-codecov,adamgreig/python-dateutil,emsoftware/python-dateutil,Bachmann1234/dateutil,mjschultz/dateutil,abalkin/dateutil