Fix TypeError: cannot unpack non-iterable PDFObjRef object, when unpacking the value of 'DW2'

[EucliTs0]

An error is occured when the 'DW2' key contains a PDFObjRef object instead of a list of int values, e.g: 'DW2': PDFObjRef:152.
To solve this issue, we utilise the resolve1() function

See: #518

Pull request

Thanks for improving pdfminer.six! Please include the following information to
help us discuss and merge this PR:

• A description of why this PR is needed. What does it fix? What does it
  improve?
• A summary of the things that this PR changes.
• Reference the issues that this PR fixes (use the fixes #(issue nr) syntax).
  If this PR does not fix any issue, create the issue first and mention that
  you are willing to work on it.

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide
instructions so we can reproduce. Include an example pdf if you have one.

Checklist

• I have added tests that prove my fix is effective or that my feature
  works
• I have added docstrings to newly created methods and classes
• I have optimized the code at least one time after creating the initial
  version
• I have updated the README.md or I am verified that this
  is not necessary
• I have updated the readthedocs documentation or I
  verified that this is not necessary
• I have added a consice human-readable description of the change to
  CHANGELOG.md

[pietermarsman]

Thanks! Could you go over the checklist? E.g. add a line to the changleog.

[EucliTs0]

Thanks! Could you go over the checklist? E.g. add a line to the changleog.

Sure, done!

[pietermarsman]

Assuming the cicd error will be fixed because I reverted the change that caused the error #534

[pietermarsman]

@EucliTs0 Thanks!