fix: Update Python tutorial sources to comply with CSP

[ittuann]

Purpose

When visiting the Python docs tutorial, images fail to load due to the current Content Security Policy (CSP) configuration.

For example, when opening https://streampipes.apache.org/docs/docs/python/latest/tutorials/1-introduction-to-streampipes-python-client/ the browser console shows the following error, which cause external images fail to load in the tutorials.

1-introduction-to-streampipes-python-client/:2740 Refused to load the image 'https://raw.githubusercontent.com/apache/streampipes/dev/streampipes-client-python/docs/img/tutorial-preparation.gif' because it violates the following Content Security Policy directive: "default-src 'self' data: blob: 'unsafe-inline' 'unsafe-eval' https://www.apachecon.com/ https://www.communityovercode.org/ https://*.apache.org/ https://apache.org/ https://*.scarf.sh/". Note that 'img-src' was not explicitly set, so 'default-src' is used as a fallback.

Remarks

PR introduces (a) breaking change(s): <yes/no>
no
PR introduces (a) deprecation(s): <yes/no>
no