title: Apache Airflow initdb command fails, due to syntax error
body: <p>I have created virtualenv for python3 using:
  <code>virtualenv -p $(which python3) ENV</code></p>
  
  <p>Then activate the source
  <code>source /Users/myusername/ENV/bin/activate</code></p>
  
  <p>Install the apache-airflow:</p>
  
  <pre><code>pip install apache-airflow
  </code></pre>
  
  <p>then <code>which airflow</code> yields <code>/Users/myusername/ENV/bin/airflow</code>
  But when I try to initdb using:</p>
  
  <pre><code>airflow initdb
  </code></pre>
  
  <p>I get below error:</p>
  
  <pre><code>    {db.py:350} INFO - Creating tables
  INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
  INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
  WARNI [airflow.utils.log.logging_mixin.LoggingMixin] cryptography not found - values will not be stored encrypted.
  ERROR [airflow.models.DagBag] Failed to import: /Library/Python/2.7/site-packages/airflow/example_dags/example_http_operator.py
  Traceback (most recent call last):
    File "/Library/Python/2.7/site-packages/airflow/models/__init__.py", line 413, in process_file
      m = imp.load_source(mod_name, filepath)
    File "/Library/Python/2.7/site-packages/airflow/example_dags/example_http_operator.py", line 27, in &lt;module&gt;
      from airflow.operators.http_operator import SimpleHttpOperator
    File "/Library/Python/2.7/site-packages/airflow/operators/http_operator.py", line 21, in &lt;module&gt;
      from airflow.hooks.http_hook import HttpHook
    File "/Library/Python/2.7/site-packages/airflow/hooks/http_hook.py", line 23, in &lt;module&gt;
      import tenacity
    File "/Library/Python/2.7/site-packages/tenacity/__init__.py", line 375, in &lt;module&gt;
      from tenacity.tornadoweb import TornadoRetrying
    File "/Library/Python/2.7/site-packages/tenacity/tornadoweb.py", line 24, in &lt;module&gt;
      from tornado import gen
    File "/Library/Python/2.7/site-packages/tornado-6.0.3-py2.7-macosx-10.14-intel.egg/tornado/gen.py", line 126
      def _value_from_stopiteration(e: Union[StopIteration, "Return"]) -&gt; Any:
                                     ^
  SyntaxError: invalid syntax
  Done.
  (ENV) ---------------------------------------------------------
  </code></pre>
  
  <p>Seems like example scripts use python 2.7 and it can't recognize the function definition syntax. 
  Does apache-airflow package need to be fixed by the next release or I can do something to fix this?
  I tried fixing this: 
  Use python2.7 instead of python3 
  then install airflow on default python 2.7 enabled on mac but this throws other errors like package "six" is not compatible.</p>