pgAdmin4 - cannot run on Debian 8

myuser(at)mydebian8:~/pgadmin4-1.0/runtime$ ./pgAdmin4
Python path: ""
Python Home: ""
Webapp path: "/home/myuser/pgadmin4-1.0/web/pgAdmin4.py"
Traceback (most recent call last):
File "/home/myuser/pgadmin4-1.0/web/pgAdmin4.py", line 24, in <module>
from pgadmin import create_app
File "/home/myuser/pgadmin4-1.0/web/pgadmin/__init__.py", line 17, in
<module>
from flask import Flask, abort, request, current_app
ImportError: No module named flask
"Failed to launch the application server, server thread exiting."

Already tried to set PYTHONPATH with export PYTHONPATH=/usr/bin/python or
whatever value but stills get the same message as above. "Python path" and
"Python Home" stills empty.

How can I fix this?

- Version of pgAdmin: pgAdmin4-1.0

- The configure options used: qmake CONFIG+=debug and qmake CONFIG+=release

- Operating system: Debian Jessie 8.6

- Whether you're running in server or desktop mode: Desktop

- Qt version and configure options used (if compiled from source): apt-get
install qt4-default

- The Python version and configure options used (if running in server
mode): Python 2.7.9 (default, Jun 29 2016, 13:08:31)

- Steps required to reproduce the problem
apt-get install qmake qt4-qmake python-dev qt4-qtconfig qt4-default
cd $PGADMIN4_SRC/runtime
qmake
make
./pgAdmin4

Python path: ""
Python Home: ""
Webapp path: "/home/myuser/pgadmin4-1.0/web/pgAdmin4.py"
Traceback (most recent call last):
File "/home/myuser/pgadmin4-1.0/web/pgAdmin4.py", line 24, in <module>
from pgadmin import create_app
File "/home/myuser/pgadmin4-1.0/web/pgadmin/__init__.py", line 17, in
<module>
from flask import Flask, abort, request, current_app
ImportError: No module named flask
"Failed to launch the application server, server thread exiting."