Apache Superset virtual env installation issue with flask
You'd want to try to first pip install --upgrade setuptools pip (this will update pip - Python's package installer)  and then pip install -r requirements.txt, which should install all packages needed for superset.
I am trying to install superset in virtual environment and running in python3.6.
pip install superset
The installation failed with the below error message
ERROR: flask-jwt-extended 3.21.0 has requirement Flask>=1.0, but you'll have flask 0.12.4 which is incompatible.
So, I upgraded the flask by
pip install --upgrade flask
which upgrade to the latest version.
After upgrading flask, I again tried installing superset. This time I got the same error. 
Not only it complained about the flask but also the process uninstalled the flask.
ERROR: flask-jwt-extended 3.21.0 has requirement Flask>=1.0, but you'll have flask 0.12.4 which is incompatible.
....
  Found existing installation: Click 7.0
    Uninstalling Click-7.0:
      Successfully uninstalled Click-7.0
  Running setup.py install for pycparser ... done
  Found existing installation: Flask 1.1.1
    Uninstalling Flask-1.1.1:
      Successfully uninstalled Flask-1.1.1