Epicbox2

* Updated dependencies
* Fixed use of deprecated packages
* Added `create_session` context manager with `exec` method

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Changes
^^^^^^^

* Added ``processes`` limits option to limit the number of processes that a sandbox can run.
For example, it can be used to prevent a fork bomb or do not allow the sandboxed code to create
new processes. It's unlimited by default.
`9 <https://github.com/StepicOrg/epicbox/pull/9>`_

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Breaking changes
^^^^^^^^^^^^^^^^

* Migrated from the ``docker-py`` Docker library to ``docker`` version ``>=2``.
* ``epicbox.create``, ``epicbox.start``, ``epicbox.destroy`` now return and accept a ``Sandbox``
object instead of a low-level container ``dict`` structure.
* Removed the obsolete ``base_workdir`` argument from ``epicbox.configure``.

Changes
^^^^^^^

* Unpinned and bumped versions of dependency packages in ``pyproject.toml`` and ``requirements.txt``.

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* Initial release on PyPI. (`5 <https://github.com/StepicOrg/epicbox/issues/5>`_)
* Fix docker volume cleanup if an exception is raised during its usage.