Fix bad import on the patch tests

tests/sources/patch.py

@@ -3,7 +3,7 @@ import pytest
 
 from buildstream._exceptions import ErrorDomain, LoadErrorReason
 from tests.testutils import cli, filetypegenerator
-from collections import contextmanager
+from contextlib import contextmanager
 
 DATA_DIR = os.path.join(
     os.path.dirname(os.path.realpath(__file__)),