core.common: move away import related stuff to my.core.utils.imports

moving without backward compatibility, since it's extremely unlikely they are used for any external modules

in fact, unclear if these methods still have much value at all, but keeping for now just in case

my/core/utils/itertools.py

@@ -105,12 +105,13 @@ else:
 
 
 def test_listify() -> None:
+    from ..compat import assert_type
+
     @listify
     def it() -> Iterator[int]:
         yield 1
         yield 2
 
     res = it()
-    from typing_extensions import assert_type  # TODO move to compat?
     assert_type(res, List[int])
     assert res == [1, 2]