This has been brought up before multiple times (here’s an easy-to-find recent instance: Allowing Multiple Versions of Same Python Package in PYTHONPATH - #9 by uranusjr).
I’ll note that easy_install does provide such functionality as well and… is on the path of deprecation because no one used it enough to justify maintaining that tool. 
You might not have encountered it, but it certainly happens a lot. The output line for users hitting dependency conflicts with apt is “The following packages have unmet dependencies:”. I’m certain that a web search will find more than a hundred thousand results for that term.
I’ll note that this happens even though Debian is curating/managing/patching the set of packages in the central repository. This is unlike Python, where anyone can upload packages to PyPI.