Add force_ci kwarg to requires_import pytest marker to allow tests
to run unconditionally in CI (GitHub Actions), even if the module
is not detected locally. Refactor autobpm test to use this instead
of manual env-checking at module level.
Add a `requires_import` pytest marker and `is_importable` fixture to
conditionally skip or adjust assertions based on whether optional
dependencies are available. Apply this to `langdetect`-dependent
language detection tests in lyrics and migration test suites.
Create a centralised pytest fixture to provide controllable stdin and
captured stdout in all tests. Simplify DummyIO/DummyIn and remove the
custom DummyOut implementation and make use of pytest builtin fixtures.
Create a centralised pytest fixture to provide controllable stdin and
captured stdout that can be applied to any tests, regardless whether
they are based on pytest or unittest.
* `io` fixture can be used as a fixture in pytest-based tests
* `IOMixin` can be used to attach `io` attribute to any test class,
including `unittest.TestCase`
