It also enables to remove the version restriction on psycopg2.

Related to swh/infra/sysadm-environment#4772

psycopg2 2.9 release broke compatibility with django 2.2 release
and deposit tests are failing since.

That error was missed by psycopg2 maintainers as they are testing
against django >= 3.1 where the issue does not appear.

So pin psycopg2 version to avoid test failures.

This will allow deposit loader to reference metadata with their correct deposit
version.

For this we:

- ensure we create the DepositRequest with str as raw_metadata (instead
  of a bytes), and
- make atom_dataset return a str instead of a bytes

  Every time this chunk of data is sent on the wire (http req to the django app)
  this needs to be an str, otherwise we have things like:

    raw_metadata = "b'[...]'"

  especially when testing against django 2, which is the only django version
  available on debian sid, so this is needed to make it possible to build the
  deb file on sid *with* tests enabled.
- add a py3-django2 environment in tox
- bump the dependency to django <3 in requirements-server.txt (otherwise it
  will take precendence on the deps: field of the tox file, thus execute tests
  in py3-django2 with django 1.11).

Note that no other tests than executing pytest have been done with django2, so
do not expect it works flawlessly.

Django 2.0 removed `django.core.urlresolvers` (which was
replaced/deprecated in favor of `django.urls` in 1.10).
Django_nose is also needed to run tests.

This unifies the way we parse xml server side and client side.

Related D396#inline-1961

This takes care of current limitations:
- duplicated elements are stored as list (and no longer dropped)
- drop extra namespace in key names
- it's generic

Close T1131

Related T882

Related T919

Also remove unused dependencies