title: Apache Airflow : airflow initdb throws ModuleNotFoundError: No module named 'werkzeug.wrappers.json'; 'werkzeug.wrappers' is not a package error
body: <p>On Ubuntu 18.04 with python 3.6.8, trying to install airflow. When I ran airflow initdb command, below error is thrown</p>
  
  <pre><code>Traceback (most recent call last):
    File "/home/uEnAFpip/.virtualenvs/airflow/bin/airflow", line 21, in &lt;module&gt;
      from airflow import configuration
    File "/home/uEnAFpip/.virtualenvs/airflow/lib/python3.6/site-packages/airflow/__init__.py", line 40, in &lt;module&gt;
      from flask_admin import BaseView
    File "/home/uEnAFpip/.virtualenvs/airflow/lib/python3.6/site-packages/flask_admin/__init__.py", line 6, in &lt;module&gt;
      from .base import expose, expose_plugview, Admin, BaseView, AdminIndexView  # noqa: F401
    File "/home/uEnAFpip/.virtualenvs/airflow/lib/python3.6/site-packages/flask_admin/base.py", line 6, in &lt;module&gt;
      from flask import Blueprint, current_app, render_template, abort, g, url_for
    File "/home/uEnAFpip/.virtualenvs/airflow/lib/python3.6/site-packages/flask/__init__.py", line 20, in &lt;module&gt;
      from .app import Flask
    File "/home/uEnAFpip/.virtualenvs/airflow/lib/python3.6/site-packages/flask/app.py", line 69, in &lt;module&gt;
      from .wrappers import Request
    File "/home/uEnAFpip/.virtualenvs/airflow/lib/python3.6/site-packages/flask/wrappers.py", line 14, in &lt;module&gt;
      from werkzeug.wrappers.json import JSONMixin as _JSONMixin
  ModuleNotFoundError: No module named 'werkzeug.wrappers.json'; 'werkzeug.wrappers' is not a package
  </code></pre>
  
  <p>tried pip3 install --upgrade Flask</p>