add getnewargs_ex to Enum, fix pickling

[benedikt-bartscher]

Summary

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
  • This change has an associated test.
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Gobot1234]

Thanks for this, please can you add a regression test that uses this?

[benedikt-bartscher]

@Gobot1234 I added a small regression test