SandboxRepositoryBackend: move imports out of destructor (#5316)

The import `from aiida.common.folders import SandboxFolder` was added to
multiple functions and is added to the top of the module. This is not
only best-practice in terms of reduction of code deduplication, in the
case of its use in the `__del__` destructor, it also fixes a bug.

When the destructor of the class is called, the interpreter may already
have cleaned up certain things, for example parts of the `sys` module.
This will make importing fail and one is greated with the exception:

    ImportError: sys.meta_path is None, Python is likely shutting down

By moving the import to the top of the module, this is no longer the
case.

aiida/repository/backend/sandbox.py

@@ -6,6 +6,8 @@
 from typing import BinaryIO, Iterable, Iterator, List, Optional, Tuple
 import uuid
 
+from aiida.common.folders import SandboxFolder
+
 from .abstract import AbstractRepositoryBackend
 
 __all__ = ('SandboxRepositoryBackend',)
@@ -15,7 +17,6 @@ class SandboxRepositoryBackend(AbstractRepositoryBackend):
     """Implementation of the ``AbstractRepositoryBackend`` using a sandbox folder on disk as the backend."""
 
     def __init__(self):
-        from aiida.common.folders import SandboxFolder
         self._sandbox: Optional[SandboxFolder] = None
 
     def __str__(self) -> str:
@@ -51,14 +52,11 @@ def initialise(self, **kwargs) -> None:
     @property
     def is_initialised(self) -> bool:
         """Return whether the repository has been initialised."""
-        from aiida.common.folders import SandboxFolder
         return isinstance(self._sandbox, SandboxFolder)
 
     @property
     def sandbox(self):
         """Return the sandbox instance of this repository."""
-        from aiida.common.folders import SandboxFolder
-
         if self._sandbox is None:
             self._sandbox = SandboxFolder()