在将sql_alchemy_conn设置为sybase连接字符串之后，initdb命令失败，并出现以下错误。是因为apache-airflow不支持Sybase作为元数据数据库吗？
airflow initdb
...
...
{db.py:222} INFO - Creating tables
Traceback (most recent call last):
File "/opt/anaconda/bin/airflow", line 15, in <module>
args.func(args)
File "/opt/anaconda/lib/python3.4/site-packages/airflow/bin/cli.py", line 524, in initdb
db_utils.initdb()
File "/opt/anaconda/lib/python3.4/site-packages/airflow/utils/db.py", line 106, in initdb
upgradedb()
File "/opt/anaconda/lib/python3.4/site-packages/airflow/utils/db.py", line 230, in upgradedb
command.upgrade(config, 'heads')
File "/opt/anaconda/lib/python3.4/site-packages/alembic/command.py", line 174, in upgrade
script.run_env()
File "/opt/anaconda/lib/python3.4/site-packages/alembic/script/base.py", line 416, in run_env
util.load_python_file(self.dir, 'env.py')
File "/opt/anaconda/lib/python3.4/site-packages/alembic/util/pyfiles.py", line 93, in load_python_file
module = load_module_py(module_id, path)
File "/opt/anaconda/lib/python3.4/site-packages/alembic/util/compat.py", line 79, in load_module_py
mod = imp.load_source(module_id, path, fp)
File "/opt/anaconda/lib/python3.4/site-packages/airflow/migrations/env.py", line 74, in <module>
run_migrations_online()
File "/opt/anaconda/lib/python3.4/site-packages/airflow/migrations/env.py", line 65, in run_migrations_online
compare_type=COMPARE_TYPE,
File "<string>", line 8, in configure
File "/opt/anaconda/lib/python3.4/site-packages/alembic/runtime/environment.py", line 773, in configure
opts=opts
File "/opt/anaconda/lib/python3.4/site-packages/alembic/runtime/migration.py", line 159, in configure
return MigrationContext(dialect, connection, opts, environment_context)
File "/opt/anaconda/lib/python3.4/site-packages/alembic/runtime/migration.py", line 103, in __init__
self.impl = ddl.DefaultImpl.get_by_dialect(dialect)(
File "/opt/anaconda/lib/python3.4/site-packages/alembic/ddl/impl.py", line 65, in get_by_dialect
return _impls[dialect.name]
KeyError: 'sybase'