One repository, multiple projects, separate interpreter for each project

I'm using Pycharm and got a monolith git repository which contains multiple projects. 

Projects could be separated in root dir like:

A/
  requirement.txt
B/
  requirement.txt
C/
  requirement.txt

but also each project could have subprojects like
A/
  requirement.txt
   a1/
     requirement.txt
   a2/ 
     requirement.txt

The problem is that each project could have own requirement.txt file and even own python version and own virtualenv, therefore I need to configure multiple interpreters, at the same time it's still a monolith repo and I should be able to push all the changes to git. Some projects could have dependencies on each other.

Does anyone have an idea how to do it Pycharm?

Just to create a separate project in pycharm or there is some other, smarter way?