[CMake] Allow Python discovery from virtual environment

[hamidelmaazouz]

trafficstars

Currently the find_package(Python3) call binds with the system Python interpreter which does not necessarily have psutil needed to run integration tests (using LLVM LIT)

Installing psutil in the system Python environment is one workaround but absolutely not recommended. The hint added to CMake in this PR allows (the possibility of) discovery of Python when the virtual environment (via virtualenv or conda) is activated.