Change to SPDX license tracking

src/ocrmypdf/extra_plugins/__init__.py

@@ -0,0 +1,3 @@
+# SPDX-FileCopyrightText: 2022 James R. Barlow
+#
+# SPDX-License-Identifier: MPL-2.0

src/ocrmypdf/extra_plugins/semfree.py

@@ -1,10 +1,5 @@
-# © 2021 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
 """Semaphore-free alternate executor.
 
 There are two popular environments that do not fully support the standard Python