Revert "Looks like the circular dependency has been fixed." ...

while it works with the tests, a real application still triggered it.

This reverts commit 150dc37.

source/fab/tools/psyclone.py

@@ -8,12 +8,17 @@
 """
 
 from pathlib import Path
-from typing import Callable, List, Optional, Union
+from typing import Callable, List, Optional, TYPE_CHECKING, Union
 
-from fab.build_config import BuildConfig
 from fab.tools.category import Category
 from fab.tools.tool import Tool
 
+if TYPE_CHECKING:
+    # TODO 314: see if this circular dependency can be broken
+    # Otherwise we have a circular dependency:
+    # BuildConfig needs ToolBox which imports __init__ which imports this
+    from fab.build_config import BuildConfig
+
 
 class Psyclone(Tool):
     '''This is the base class for `PSyclone`.