Enable W191 and W291 flake8 checks.

Remove trailing whitespace from Python files.
Convert tabs to spaces.

contrib/devtools/copyright_header.py

@@ -506,7 +506,7 @@ def file_has_hashbang(file_lines):
 
 def insert_python_header(filename, file_lines, start_year, end_year):
     if file_has_hashbang(file_lines):
-        insert_idx = 1 
+        insert_idx = 1
     else:
         insert_idx = 0
     header_lines = get_python_header_lines_to_insert(start_year, end_year)
@@ -571,7 +571,7 @@ def insert_cmd(argv):
     if extension not in ['.h', '.cpp', '.cc', '.c', '.py']:
         sys.exit("*** cannot insert for file extension %s" % extension)
 
-    if extension == '.py': 
+    if extension == '.py':
         style = 'python'
     else:
         style = 'cpp'