Add SPDX short license identifier to all Python files

See: https://spdx.github.io/spdx-spec/appendix-V-using-SPDX-short-identifiers-in-source-files/

tests/test_check.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: GPL-3.0-only
+
 import pandas as pd
 from colorama import Fore
 

tests/test_fix.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: GPL-3.0-only
+
 import csv_metadata_quality.fix as fix