[MNT] test for isolation of developer dependencies, and basic pytest-less test for BaseObject

[fkiraly]

This adds basic tests to ensure isolation from developer dependencies including pytest outside of test modules, and adds a basic pytest-less test for BaseObject.

The approach is as follows:

• addition of a new GHA job
• only skbase plain is installed, no dev dependencies
• a test file _nopytest_tests is added, which can contain pytest-free checks
• all_objects plain is executed. As this crawls all modules (except tests and a few others), it will spot unisolated developer dependency imports
• parameterless methods of BaseObject are executed on an instance. This ensures those methods are isolated, and it would also catch the issue with clone described here: [BUG] clone breaks when skbase is installed without pytest #177