Revert "Allowing whitespace-only lines."

This makes lint.py's behavior match pre-commit, which is useful because lint.py
errors show up inline on GitHub PRs.

This reverts commit ebb3dc4

ci/lint.py

@@ -97,7 +97,7 @@ class TrailingWhitespaceLinter(Linter):
     msg = 'Contains trailing whitespace'
     def check_line(self, line):
         line = line.replace('\r', '').replace('\n', '')
-        return not line.strip() or line == line.rstrip('\t ')
+        return line == line.rstrip('\t ')
     def fix_line(self, line):
         return line.rstrip('\t ')