Pytest-invenio

- Fixes an import so that pytest-invenio is now usable without
Invenio-Accounts installed.

- Adds new UserFixture for easier test user creation.

- Adds support for using importlib_metadata to read the patched entry points.

- Add APP_THEME and THEME_ICONS in default app config, often needed when testing
invenio packages that will render templates.

- Remove pytest-celery because it's still an alpha release.

- BACKWARD INCOMPATIBLE: Changes to use isort, pycodestyle and pydocstyle via
pytest plugins. You need to update `pytest.ini` and remove the ``--pep8``
from the addopts and instead add ``--isort --pydocstyle --pycodestyle``:

.. code-block:: ini

addopts = --isort --pydocstyle --pycodestyle ...

In `./run-tests.sh` script you should also remove calls to pydocstyle and
isort as both are now integrated with pytest.

- BACKWARD INCOMPATIBLE: Upgrade dependencies: coverage, pytest-flask,
check-manifest, pytest. You need to set the pytest-flask live server
fixture scope in your pytest config:

.. code-block:: ini

[pytest]
live_server_scope = function

- Decommission pytest-pep8 (last release in 2014) in favour of pycodestyle.