update/cleanup Python dev scripts

- put common code in its own util.py file
- same Python3 shebang for all scripts
- add file docstrings
- fix format string replacement fields in man page template

scripts/create_test_data.py

@@ -9,13 +9,9 @@
 
 """Create testdata for extractor tests"""
 
-import sys
-import os.path
 import argparse
 
-ROOTDIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
-sys.path.insert(0, os.path.realpath(ROOTDIR))
-
+import util  # noqa
 from gallery_dl import extractor
 from test.test_results import ResultJob, setup_test_config