Consider not using "http" anymore

[buhtz]

Hello,

readme_renderer/readme_renderer/clean.py

Line 84 in 71f7cba

url_schemes={"http", "https", "mailto"},

Today it might be a good time to remove the support of "http" in readme_renderer because of security issues.
Maybe there could be a config option to opt-in it again if needed.
And before doing this there should e deprecation warning period of course