Fix last ambigious isinstance
Fix last ambigious isinstance

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