Pytest-bdd-ng

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This release introduces breaking changes, please refer to the `Migration from 4.x.x`.

- Rewrite the logic to parse Examples for Scenario Outlines. Now the substitution of the examples is done during the parsing of Gherkin feature files. You won't need to define the steps twice like ``given("there are <start> cucumbers")`` and ``given(parsers.parse("there are {start} cucumbers"))``. The latter will be enough.
- Removed ``example_converters`` from ``scenario(...)`` signature. You should now use just the ``converters`` parameter for ``given``, ``when``, ``then``.
- Removed ``--cucumberjson-expanded`` and ``--cucumber-json-expanded`` options. Now the JSON report is always expanded.
- Removed ``--gherkin-terminal-reporter-expanded`` option. Now the terminal report is always expanded.

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- `when` and `then` steps now can provide a `target_fixture`, just like `given` does. Discussion at https://github.com/pytest-dev/pytest-bdd/issues/402.
- Drop compatibility for python 2 and officially support only python >= 3.6.
- Fix error when using `--cucumber-json-expanded` in combination with `example_converters` (marcbrossaissogeti).
- Fix `--generate-missing` not correctly recognizing steps with parsers

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- Fix a bug that prevents using comments in the ``Examples:`` section. (youtux)

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- Fixed performance regression introduced in 4.0.0 where collection time of tests would take way longer than before. (youtux)

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This release introduces breaking changes, please refer to the `Migration from 3.x.x`.

- Strict Gherkin option is removed (``scenario()`` does not accept the ``strict_gherkin`` parameter). (olegpidsadnyi)
- ``scenario()`` does not accept the undocumented parameter ``caller_module`` anymore. (youtux)
- Given step is no longer a fixture. The scope parameter is also removed. (olegpidsadnyi)
- Fixture parameter is removed from the given step declaration. (olegpidsadnyi)
- ``pytest_bdd_step_validation_error`` hook is removed. (olegpidsadnyi)
- Fix an error with pytest-pylint plugin 374. (toracle)
- Fix pytest-xdist 2.0 compatibility 369. (olegpidsadnyi)
- Fix compatibility with pytest 6 ``--import-mode=importlib`` option. (youtux)

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- Parse multiline steps according to the gherkin specification 365.