python3: implement __dir__ for BaseObject and PagesBrowser

Since those classes store some class-level declared fields (Field and
URL) in a dictionary and are not present as attributes, those fields are
not present in dir() even though __getattr__ will find them.
By reimplementing __dir__, they can be discovered as ordinary
attributes.
Status Job ID Name Coverage
  Build
failed #2253
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  Test
skipped #2254
lint
skipped #2255
unittests
 
  Deploy
skipped #2256
doc
 
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