setup.py: only require enum34 for python older than 3.4

This fixes errors like:

    File "python3.6/lib/python3.6/re.py", line 142, in <module>
      class RegexFlag(enum.IntFlag):
  AttributeError: module 'enum' has no attribute 'IntFlag
10 jobs for fix/python3.6 in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Jerome.Python3

01:05:43

canceled SlapOS.SoftwareReleases.IntegrationTest-Jerome-Python3

26:34:58

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP.own-distributor

87:15:58

running SlapOS.Eggs.UnitTest-Jerome.Python3

42215:44:04

running SlapOS.Eggs.UnitTest-Jerome.Python3

42215:37:16

running SlapOS.Eggs.UnitTest-Jerome.Python3

42215:29:32

running SlapOS.Eggs.UnitTest-Jerome.Python3

42215:16:12

running SlapOS.Eggs.UnitTest-Jerome.Python3

42210:46:57

passed SlapOS.Eggs.UnitTest-Jerome.Python3

04:02:30

passed SlapOS.Eggs.UnitTest-Jerome.Python3