PKG: Declare Pillow as optional dependency

[MartinThoma]

Closes #1390

[MartinThoma]

@MasterOdin What do you think about this? I think (hope) that we don't get more dependencies, but with full people would have a clear way how to install all dependencies.

I don't see a way around the PIL / cryptodome dependencies if we want to have those features. And I think those features add enough value to justify an optional dependency.

I am uncertain about the name "full". Maybe "all" would be better?

[MasterOdin]

Should include a link on https://github.com/py-pdf/PyPDF2/blob/main/docs/user/extract-images.md back to the installation page referencing that need to install [image].

May also want to update https://pypdf2.readthedocs.io/en/latest/modules/PageObject.html#PyPDF2._page.PageObject.images to include that reference as well?

[codecov]

Codecov Report

Base: 94.19% // Head: 94.19% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (9d4a350) compared to base (cc55321).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1392   +/-   ##
=======================================
  Coverage   94.19%   94.19%           
=======================================
  Files          28       28           
  Lines        5082     5085    +3     
  Branches      968      968           
=======================================
+ Hits         4787     4790    +3     
  Misses        176      176           
  Partials      119      119           

Impacted Files	Coverage Δ
PyPDF2/_page.py	92.16% <ø> (ø)
PyPDF2/filters.py	97.28% <100.00%> (+0.02%)	⬆️

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[MasterOdin]

Additionally, I think that a try/except should be added around:

https://github.com/py-pdf/PyPDF2/blob/cc553210524a8009c91409b76f05912469cfa927/PyPDF2/filters.py#L567

where we do something like:

try:
    from PIL import Image
except ImportError:
    raise DependencyError("pillow is required to do image extraction")

This is similar to what we do with PyCryptodome in _encryption.py. However, maybe also append something like ", install with pip install PyPDF2[image]" or a link to the installation guide for it?

We definitely do need to add something, as shown in the case for the user in #1390, they'll get an error about PIL import failing, and then they might try to install PIL vs pillow.

[MasterOdin]

If wanted a test here, could add a test with monkeypatch.setitem modifying sys.modules to remove it from the environment. See https://stackoverflow.com/a/28361013/4616655 for an example.

[MartinThoma]

That's an awesome idea! Thanks for sharing it - I've written the test!