Fix tests on Python 3.8

__file__ used to not be absolute, so we need to explicitly make it so.

Author: PeterJCLaw

tests.py

@@ -10,7 +10,7 @@
 from pathlib import Path
 
 TESTS = Path(__file__).parent / 'tests'
-VALIDATE_GENERATED_FILES = Path(__file__).parent / 'validate-generated-files.py'
+VALIDATE_GENERATED_FILES = Path(__file__).absolute().parent / 'validate-generated-files.py'
 
 EXPECTED = [
     (TESTS / 'ok', 0),