Better image resizing

[satyamtg]

Contains the following changes -

• added argument allow_upscaling to upscale image if it cannot be resized due to smaller dimentions than required
• use src instead of png_image in tests
• use _ for unused variables in tests
• add tests for allow_upscaling
• convert image to RGB if saving as JPEG

Also, this would fix https://github.com/openzim/youtube/issues/104

This is compatible with the previous API but contains an extra option allow_upscaling

[codecov[bot]]

Codecov Report

Merging #25 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #25   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines          448       467   +19     
=========================================
+ Hits           448       467   +19     

Impacted Files	Coverage Δ
src/zimscraperlib/imaging.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update ad7e593...11eb5d0. Read the comment docs.