Pytest-steps

- New fixtures `step_bag` and `cross_bag`, that may be used when `pytest-harvest` is installed. These fixtures are versions of `pytest-harvest`'s `results_bag` fixture that can be used with steps. Fixes [49](https://github.com/smarie/python-pytest-steps/issues/49). PR [#46](https://github.com/smarie/python-pytest-steps/pull/46) by [`j-carson`](https://github.com/j-carson).

- packaging improvements: set the "universal wheel" flag to 1, and cleaned up the `setup.py`. In particular removed dependency to `six` for setup and added `py.typed` file, as well as set the `zip_safe` flag to False. Removed tests folder from package. Fixes [39](https://github.com/smarie/python-pytest-steps/issues/39)

Removed import warning. Fixed [37](https://github.com/smarie/python-pytest-steps/issues/37)

Added `pyproject.toml`.

`test_steps` can now be used on test functions located inside classes. Fixed [16](https://github.com/smarie/python-pytest-steps/issues/16)

**Warning**: as a consequence of the fix above, the order of arguments has changed. this has an impact for manual execution. See [here](https://smarie.github.io/python-pytest-steps/#d-calling-decorated-functions-manually) for details.

1.6.4 - python 2 bugfix

Fixed issue happening with python 2 when `unicode_literals` are used in the parameters receiving string types. Fixed [34](https://github.com/smarie/python-pytest-steps/issues/34)

Added `__version__` attribute at package level