ERP5Form,Formulator: make widget a new style class

This makes consistent method resolution order with python3 and reveal a
problem that relation field widgets render_pdf was not the expected one
on python3, solve this by defining explicitly the method.
19 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

01:04:56

failed ERP5.UnitTest-Master

01:35:17

passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python3

00:19:52

passed Wendelin.UnitTest-Master

01:23:42

passed ERP5.CodingStyleTest-Master

01:36:32

passed ERP5.CodingStyleTest-Master

00:48:05

failed ERP5.UnitTest-Master

01:34:58

failed ERP5.UnitTest-Master

02:40:55

passed SlapOS.Eggs.UnitTest-Master.Python2

00:15:51

passed SlapOS.Eggs.UnitTest-Master.Python2

00:16:30

passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python2

00:14:11

passed SlapOS.Eggs.UnitTest-Master.Python3

00:20:34

passed SlapOS.Eggs.UnitTest-Master.Python3

00:15:56

passed SlapOS.Eggs.UnitTest-Master.Python3

00:15:40

passed SlapOS.Eggs.UnitTest-Master.Python3

00:18:05

passed Wendelin.UnitTest-Master

00:39:19

passed Wendelin.UnitTest-Master

01:12:32