Move conftest to top-level

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