Drop old test adjustments, we only test DateTime >= 3 nowadays.

Make the condition for test_pickle_old_with_micros_as_float based on
python version for simplicity.

redefine-in-handler does not exist on pylint 3 and was no longer emitted here
on python2.

Because the librairies used here were never ported to python3.

Notable changes:

  - ERP5Site_createGoogleUserToOAuth is dropped
  - internal API changed radically, so customizations made by overriding
    scripts are broken.
  - the core logic is now implemented in a connector class (still in
    portal_oauth for simplicity, but it would be simpler to move it to
    portal_web_services)

No changes required in the google console, the redirect uri is still
ERP5Site_receiveGoogleCallback

The `details` part of the output of functional test cases was
being wrongly converted to an unicode string in Python 2.
This caused problems when the output was sent to a file
object not supporting non-ASCII code points.
This commit fixes that behaviour.

Some tests were added to check warnings, which were not working in
Python 2 because the warning message was shown just once.
This is fixed by adding a `warnings.simplefilter("always")` inside
the `catch_warnings` context manager, as recommended in
https://docs.python.org/3/library/warnings.html#testing-warnings .
Note that the previous filter is restored on leaving the context
manager.