Move conftest to top-level

[jtojnar]

Same as https://github.com/smarie/python-pytest-harvest/pull/62

This fixes the following error:

________________________ ERROR collecting test session _________________________
Defining 'pytest_plugins' in a non-top-level conftest is no longer supported:
It affects the entire test suite instead of just below the conftest as expected.
  /build/pytest-steps-1.8.0/pytest_steps/conftest.py
Please move it to a top level conftest file at the rootdir:
  /build/pytest-steps-1.8.0
For more information, visit:
  https://docs.pytest.org/en/stable/deprecations.html#pytest-plugins-in-non-top-level-conftest-files

[smarie]

Thanks @jtojnar !

Closing/Reopening PR to see the result of the CI