feat: isolate arch and statsmodels as soft dependencies, new all_extras soft dependency set

[fkiraly]

This PR isolates arch and statsmodels as soft dependencies, moving them to an all_extras soft dependency set.

Isolation is done as follows:

• moving imports into functions and classes
• adding a _tags variable with dependencies specified to all classes impacted
• isolating tests with pytest.skipif using skbase _check_soft_dependencies
• skipping doctests triggering the imports. Important, if this reduces coverage, we should add those doctests as pytests.
• removing or replacing type annotations that require imports

Includes changes:

• the CI test.yml is updated to include a full test run with all soft dependencies, and one without. The former is functionally identical with current, the latter is a new job called test-no-softdeps.
• the CI test.yml is updated with a job testing for full dependency isolation, test-nodevdeps, using skbase all_objects to test successful import of all modules except tests
• minor change, adds a few test skips missed in fix: temporarily skip sporadically failing tests due to LU decomposition #28

I would recommend to merge #10 and #26 first to ensure better test coverage before merging this.

Depends on #40 so CI runs, that PR should be merged first.

[fkiraly]

approved via #52