Pytest-reana

- Adds support for Python 3.9.
- Fixes minor code warnings.
- Fixes installation by upgrading REANA-DB version.

- Adds new `__reana` database schema for `db` fixture.
- Fixes a problem related to duplicated database session.
- Changes code formatting to respect `black` coding style.
- Changes documentation to single-page layout.

- Adds fixtures for secrets store.
- Centralises test requirements.
- Adds Python 3.8 support.

- Makes workspace path configurable for the `sample_workflow_workspace`
fixture through the `path` parameter.
- Adds `sample_serial_workflow_in_db` fixture.
- Exposes previously hidden `sample_yadage_workflow_in_db` fixture.
- Adds missing database session close in `session` fixture.
- Adds helpers to represent starting and requeueing job conditions,
`sample_condition_for_starting_queued_workflows` and
`sample_condition_for_requeueing_workflows`.

- Adds directory including sample workspace data.

- Initial public release.