Bugfix: copying multiple attachments

[JelleBoersma]

Copying multiple attachments using the part.copy() method caused the temporary directory to be removed.

Fixes #

Checklist:

• My code follows the pykechain style
  • I added type hinting to all functions
  • I added documentation for all public functions
  • I locally used tox to check for dists and docs errors
  • My code passes the pep8 and flake8 linting checks and no warnings
  • I removed unused imports, using Code > Optimize Imports in PyCharm
  • I used black to format the new code
• I have added tests that prove my fix is effective or that my feature works
  • I committed fresh test cassettes
  • I have proper test coverage (don't decline the coverage of the test)
• I asked another teammate to review the code
• I update the Changelog.md with the appropriate changes.