STY: Move _repr_mimebundle_

[j-t-1]

Put as the last member of PdfWriter, as is different functionality to the other methods.

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.36%. Comparing base (6d8a18a) to head (ba4ddb7).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pypdf/_writer.py	77.77%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2978      +/-   ##
==========================================
- Coverage   96.36%   96.36%   -0.01%     
==========================================
  Files          52       52              
  Lines        8751     8749       -2     
  Branches     1591     1591              
==========================================
- Hits         8433     8431       -2     
  Misses        190      190              
  Partials      128      128              

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