sqlalchemy: fix deprecation warning for dbapi classmethod (#294)

Rename `dbapi` classmethod to `import_dbapi` as required by SQLAlchemy 2

Closes #289

Signed-off-by: Jesse Whitehouse <jesse.whitehouse@databricks.com>
Signed-off-by: Sai Shree Pradhan <saishree.pradhan@databricks.com>

Author: Jesse

src/databricks/sqlalchemy/base.py

@@ -82,7 +82,7 @@ class DatabricksDialect(default.DefaultDialect):
     EMPTY_FK = EMPTY_INDEX = []
 
     @classmethod
-    def dbapi(cls):
+    def import_dbapi(cls):
         return sql
 
     def _force_paramstyle_to_native_mode(self):