Move import to the top in env.py file to avoid linter warnings (#349)

flask_migrate/templates/flask/env.py

@@ -5,6 +5,7 @@
 
 from sqlalchemy import engine_from_config
 from sqlalchemy import pool
+from flask import current_app
 
 from alembic import context
 
@@ -21,7 +22,6 @@
 # for 'autogenerate' support
 # from myapp import mymodel
 # target_metadata = mymodel.Base.metadata
-from flask import current_app
 config.set_main_option(
     'sqlalchemy.url',
     str(current_app.extensions['migrate'].db.engine.url).replace('%', '%%'))