dependency pinning for svglib to version 1.5.1

[payamnj]

Due to a recent update, svglib version 1.6.0, a dependency of easy-thumbnails[svg], now requires pycairo. This addition causes installation to fail unless the system has system-level libraries like cario. Since the svglib maintainer has acknowledged the issue and plans to fix it, the best temporary solution is to pin svglib to version 1.5.1 in your project's dependency file to prevent this new, problematic version from being installed.

reported issue in the svglib repository:
deeplook/svglib#421

[fsbraun]

@payamnj Thanks for the explanation. I've been aware of the issue but not on the path forward. Would you like to create a PR to pin svglib to !=1.6.0?

[payamnj]

@fsbraun Sure, I created a PR, but it seems that Flake8 also has some dependency conflict and the build is failing.