scripts: use explicit relative imports of contributor_data

Avoid any ambiguity ... and avoid pytype failing to find
scripts/contributor_data.py when scripts isn't in sys.path.

scripts/shortlog.py

@@ -9,7 +9,7 b' import argparse'
 import os
 from collections import Counter
 
-import contributor_data
+from . import contributor_data
 
 
 def main():

scripts/update-copyrights.py

@@ -42,7 +42,7 b' import os'
 import re
 from collections import defaultdict
 
-import contributor_data
+from . import contributor_data
 
 
 def sortkey(x):