update flask project from python2 to python3
You are using implicit relative imports (the Python 2 model); you need to use absolute references or explicit relative imports:
from .app1.app1 import mod as app1where the leading . signals that the rest is relative to the current package, or
from app.app1.app1 import mod as app1See PEP 328 - Imports: Multi-Line and Absolute/Relative.
You probably will have other issues however. Porting is not that trivial, read up on the issues in the Porting to Python 3 book. This issue is a common migration problem.