Avoid leaking "name" variable in AbstractSandbox

[Avasam]

Summary of changes

This was noticed in typeshed:
https://github.com/python/typeshed/blob/449f70f1f1bc787eb56699a5ce153c3efbfe2f4f/stubs/setuptools/%40tests/stubtest_allowlist.txt#L40-L41

Before:

>>> from setuptools.sandbox import AbstractSandbox
>>> AbstractSandbox.name
'tmpnam'

After:

>>> from setuptools.sandbox import AbstractSandbox
>>> AbstractSandbox.name
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'AbstractSandbox' has no attribute 'name'

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.
  (See documentation for details)

[abravalheri]

Thank you very much!