Change to SPDX license tracking

src/ocrmypdf/_plugin_manager.py

@@ -1,8 +1,5 @@
-# © 2020 James R. Barlow: github.com/jbarlow83
-#
-# This Source Code Form is subject to the terms of the Mozilla Public
-# License, v. 2.0. If a copy of the MPL was not distributed with this
-# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+# SPDX-FileCopyrightText: 2022 James R. Barlow
+# SPDX-License-Identifier: MPL-2.0
 
 """Plugin manager using pluggy."""