title: Airflow initdb with mysql as backend is not working
body: <p>when I initialize airflow using mysql as backend  I see below error</p>
  
  <blockquote>
  <pre><code>File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/default.py",
  </code></pre>
    
    <p>line 453, in connect
            return self.dbapi.connect(*cargs, **cparams)
          File "/usr/local/lib/python3.7/site-packages/MySQLdb/<strong>init</strong>.py", line 80,
    in Connect
            from MySQLdb.connections import Connection
          File "/usr/local/lib/python3.7/site-packages/MySQLdb/connections.py", line
    36
            raise errorclass, errorvalue
                            ^
        SyntaxError: invalid syntax</p>
  </blockquote>
  
  <pre><code>I tried re-installing mysql-python and pymysql still its same 
  
  airflow initdb
  /usr/local/lib/python3.7/site-packages/airflow/configuration.py:627: DeprecationWarning: You have two airflow.cfg files: /home/airflow/airflow/airflow.cfg and /home/airflow/airflow.cfg. Airflow used to look at ~/airflow/airflow.cfg, even when AIRFLOW_HOME was set to a different value. Airflow will now only read /home/airflow/airflow.cfg, and you should remove the other file
    category=DeprecationWarning,
  [2019-09-24 12:10:32,310] {settings.py:213} INFO - settings.configure_orm(): Using pool settings. pool_size=5, max_overflow=10, pool_recycle=1800, pid=15403
  [2019-09-24 12:10:32,508] {__init__.py:51} INFO - Using executor LocalExecutor
  DB: mysql://airdba:***@host/airflow_main
  [2019-09-24 12:10:32,820] {db.py:369} INFO - Creating tables
  Traceback (most recent call last):
    File "/usr/local/bin/airflow", line 32, in &lt;module&gt;
      args.func(args)
    File "/usr/local/lib/python3.7/site-packages/airflow/bin/cli.py", line 1102, in initdb
      db.initdb(settings.RBAC)
    File "/usr/local/lib/python3.7/site-packages/airflow/utils/db.py", line 106, in initdb
      upgradedb()
    File "/usr/local/lib/python3.7/site-packages/airflow/utils/db.py", line 377, in upgradedb
      command.upgrade(config, 'heads')
    File "/usr/local/lib/python3.7/site-packages/alembic/command.py", line 298, in upgrade
      script.run_env()
    File "/usr/local/lib/python3.7/site-packages/alembic/script/base.py", line 489, in run_env
      util.load_python_file(self.dir, "env.py")
    File "/usr/local/lib/python3.7/site-packages/alembic/util/pyfiles.py", line 98, in load_python_file
      module = load_module_py(module_id, path)
    File "/usr/local/lib/python3.7/site-packages/alembic/util/compat.py", line 173, in load_module_py
      spec.loader.exec_module(module)
    File "&lt;frozen importlib._bootstrap_external&gt;", line 728, in exec_module
    File "&lt;frozen importlib._bootstrap&gt;", line 219, in _call_with_frames_removed
    File "/usr/local/lib/python3.7/site-packages/airflow/migrations/env.py", line 92, in &lt;module&gt;
      run_migrations_online()
    File "/usr/local/lib/python3.7/site-packages/airflow/migrations/env.py", line 78, in run_migrations_online
      with connectable.connect() as connection:
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 2206, in connect
      return self._connection_cls(self, **kwargs)
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 103, in __init__
      else engine.raw_connection()
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 2306, in raw_connection
      self.pool.unique_connection, _connection
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/base.py", line 2275, in _wrap_pool_connect
      return fn()
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 303, in unique_connection
      return _ConnectionFairy._checkout(self)
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 760, in _checkout
      fairy = _ConnectionRecord.checkout(pool)
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 492, in checkout
      rec = pool._do_get()
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/impl.py", line 139, in _do_get
      self._dec_overflow()
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/langhelpers.py", line 68, in __exit__
      compat.reraise(exc_type, exc_value, exc_tb)
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/util/compat.py", line 153, in reraise
      raise value
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/impl.py", line 136, in _do_get
      return self._create_connection()
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 308, in _create_connection
      return _ConnectionRecord(self)
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 437, in __init__
      self.__connect(first_connect_check=True)
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 639, in __connect
      connection = pool._invoke_creator(self)
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/strategies.py", line 114, in connect
      return dialect.connect(*cargs, **cparams)
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/default.py", line 453, in connect
      return self.dbapi.connect(*cargs, **cparams)
    File "/usr/local/lib/python3.7/site-packages/MySQLdb/__init__.py", line 80, in Connect
      from MySQLdb.connections import Connection
    File "/usr/local/lib/python3.7/site-packages/MySQLdb/connections.py", line 36
      raise errorclass, errorvalue
                      ^
  SyntaxError: invalid syntax
  
  
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 437, in __init__
      self.__connect(first_connect_check=True)
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/pool/base.py", line 639, in __connect
      connection = pool._invoke_creator(self)
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/strategies.py", line 114, in connect
      return dialect.connect(*cargs, **cparams)
    File "/usr/local/lib/python3.7/site-packages/sqlalchemy/engine/default.py", line 453, in connect
      return self.dbapi.connect(*cargs, **cparams)
    File "/usr/local/lib/python3.7/site-packages/MySQLdb/__init__.py", line 80, in Connect
      from MySQLdb.connections import Connection
    File "/usr/local/lib/python3.7/site-packages/MySQLdb/connections.py", line 36
      raise errorclass, errorvalue
                      ^
  SyntaxError: invalid syntax
  </code></pre>