Searching for a solution of anothe problem, I found some usefull hints about pip's parameters "--allow-external" and "--allow-all-external". I did not really get the difference but saw that both are deprecated "due to changes in the repository protocol".
What is the new/ prefered way to install packages and versions that are not available by default? -- Or does pip now allows external by default?
I couldn't find any hint at the pip help or at stackoverflow.