feat: improve pdf export quality

[tahvane1]

This PR uses img2pdf to export charts to pdf. Current solutions uses compression in such way that pdf is grainy.
Using img2pdf removes this issue mostly.

SUMMARY

Current pdf quality is poor and this improves it significantly

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

img2pdf needs to be added to docker images.

ADDITIONAL INFORMATION

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[villebro]

This PR apparently adds a new requirement. I don't oppose this change, but I would propose the following:

• Add img2pdf to one of the .in files in requirements/. Based on this change it would need to go into base.in. Also do check that the license is Apache 2.0 compliant.
• Add before/after screenshots to show how quality changes.
• Add info about how the pdf file size changed before/after. Lossless compression is great, but can bloat the attachment size.

[tahvane1]

img2pdf is using GNU Lesser General Public License v3 and it seems that it is not accepted in ASF projects. So need to look for alternative approach then.

[villebro]

img2pdf is using GNU Lesser General Public License v3 and it seems that it is not accepted in ASF projects. So need to look for alternative approach then.

Dang, that's a real shame! 🙁

[rusackas]

Do we know if the low resolution is caused by the capture method (Image.open(BytesIO(snap))) or by the output method (images[0].save(...))? Apparently if it's the latter, you can add a resolution=300.0 parameter.

[tahvane1]

Resolution does not fix issue. I can get similar quality by setting quality=100 but this results to a pdf that has double the size what img2pdf produces. I will continue my testings...

[villebro]

FYI I converted this to draft. @tahvane1 please feel free to change it back to non-draft if you find a new ASF-compliant way to fix this issue.