fix: dialect atribute wasn't provided

sqlalchemy_bigquery/__init__.py

@@ -22,7 +22,7 @@
 
 from .version import __version__  # noqa
 
-from .base import BigQueryDialect
+from .base import BigQueryDialect, dialect  # noqa
 from .base import (
     STRING,
     BOOL,

sqlalchemy_bigquery/base.py

@@ -986,6 +986,8 @@ def get_view_definition(self, connection, view_name, schema=None, **kw):
         return view.view_query
 
 
+dialect = BigQueryDialect
+
 try:
     import alembic  # noqa
 except ImportError:

tests/unit/test_sqlalchemy_bigquery.py

@@ -171,3 +171,10 @@ def test_multi_value_insert(faux_conn, last_query):
         " (%(id_m0:INT64)s), (%(id_m1:INT64)s), (%(id_m2:INT64)s)",
         {"id_m0": 0, "id_m1": 1, "id_m2": 2},
     )
+
+
+def test_follow_dialect_attribute_convention():
+    import sqlalchemy_bigquery.base
+
+    assert sqlalchemy_bigquery.dialect is sqlalchemy_bigquery.BigQueryDialect
+    assert sqlalchemy_bigquery.base.dialect is sqlalchemy_bigquery.BigQueryDialect