Fix mysql db api

By default the multiline option for the db api was included in
PyMySql<0.8 but now that it is removed[1] we need to pass in the
multi=True argument to the cursor to maintain the same execution
behavior. Unfortunately, I wasn't sure on how to do that
so I modified the op.execute method to run the sql commands one
line at a time instead. It seems to work for the test cases.

[1]c0aa317940

Change-Id: If139f0941562b7a792d813b5e23a47eb92487304
Story: 2001420

subunit2sql/migrations/versions/1679b5bc102c_add_subsecond_columns_to_test_runs_table.py

@@ -43,7 +43,10 @@ def upgrade():
     migration_context = context.get_context()
     if migration_context.dialect.name == 'mysql':
         with open(sql_path, 'r') as sql_file:
-            op.execute(sql_file.read())
+            for line in sql_file.read().splitlines():
+                # don't execute empty or commented lines
+                if line and not line.startswith('--'):
+                    op.execute(line)
     else:
         op.add_column('test_runs', sa.Column('start_time_microsecond',
                                              sa.Integer(), default=0))