Add forgotten mypy.ini and requirements-test.txt files

In D2146#50293, @ardumont wrote:

Build has FAILED

mypy is unhappy, don't know why though.

  flake8: commands succeeded
ERROR:   mypy: commands failed

Just one note: the name is IMHO inappropriate. This is NOT a JSON lister. JSON is nothing but the serialization format used to retrieve some (more or less) well defined structured data.

I have no strong opinion on this. olsad's idea might be an acceptable solution.

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rebased + merge with D2096

merged in D2095

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smalle rework related to skipping a few tests due to origin id handling

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revert usage for self._test_origin_ids and use_url to fix py3-no-origin-ids

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rebae + kill print statement

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fix definition and usage of swh_contents and swh_origins fixtures

add some comment in conftest to explain a bit the postgres stuff

Reactivate [testenv:py3-no-origin-ids] in tox

use memory cls for the journal_writer instead of deprecated inmemory

also fix test_db to not depends on running tests via pifpaf

moved at the top of the stack

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xfail does not mean xerror...

also xfail test_snapshot (until it has been ported to pytest/pytest_postgresql)

rebase + fix tests in tox (update mypy.ini and requirements-test.txt)

mark test_in_memory and test_api_client as xfail

[really] add comments/docs and move the impl. of the rpc_client fixture in the plugin

add comments/docs and move the impl. of the rpc_client fixture in the plugin

rebase and rephrase the ci message