Add support to extract gray scale images

[joeywang4]

Currently, when gray scale images are extracted, they will be incorrectly transformed to RGB images. This PR fixed this issue by changing the palette and the mode when images are extracted.

[codecov]

Codecov Report

Base: 94.31% // Head: 94.01% // Decreases project coverage by -0.30% ⚠️

Coverage data is based on head (6efa8a6) compared to base (940819f).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1460      +/-   ##
==========================================
- Coverage   94.31%   94.01%   -0.31%     
==========================================
  Files          28       30       +2     
  Lines        5171     5443     +272     
  Branches      980     1038      +58     
==========================================
+ Hits         4877     5117     +240     
- Misses        177      197      +20     
- Partials      117      129      +12     

Impacted Files	Coverage Δ
PyPDF2/filters.py	97.31% <100.00%> (+0.01%)	⬆️
PyPDF2/_merger.py	91.11% <0.00%> (-6.46%)	⬇️
PyPDF2/_writer.py	88.73% <0.00%> (-2.80%)	⬇️
PyPDF2/constants.py	100.00% <0.00%> (ø)
PyPDF2/_utils.py	99.48% <0.00%> (ø)
PyPDF2/__init__.py	100.00% <0.00%> (ø)
PyPDF2/generic/_data_structures.py	95.62% <0.00%> (+0.50%)	⬆️
PyPDF2/_reader.py	90.30% <0.00%> (+0.68%)	⬆️

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[MartinThoma]

Very nice!

Do you happen to have an example pdf where the new code would be applied? Or could you create one and upload it here?

I would add it to the https://github.com/py-pdf/PyPDF2 + add a test

[joeywang4]

Ok, I have uploaded a test file grayscale.pdf and added it to test_get_images.

[MartinThoma]

Hi @joeywang4 ,

I've just added the grayscale.pdf to the sample-files git submodule. Would you be so kind to remove the file from your PR + use sample-files/019-grayscale-image/grayscale-image.pdf?

I want to avoid that the PyPDF2 repository becomes bigger and bigger due to example PDF files. This might have an impact on people who clone PyPDF2 / install from the repository. Hence adding the file via submodule.

[MartinThoma]

The code looks good - thanks for adding the test!

Please just remove the resources/grayscale.pdf and use SAMPLE_ROOT / "019-grayscale-image/grayscale-image.pdf" instead :-)

[joeywang4]

@MartinThoma I have removed the test file and updated the path. Thanks for the suggestion!
I also changed the path of the extracted image file when running the test. Otherwise, the extracted image cannot be correctly written to a file when I ran the test locally on my computer.

[MartinThoma]

Very good work! 👍

If you want, I'll add you to the contributors list :-)