W191: erroneously checks indentation within triple-quoted strings

[petere]

The following code

if True:
    print("""
    tab at start of this line
""")

produces

test.py:3:1: E101 indentation contains mixed spaces and tabs
test.py:3:1: W191 indentation contains tabs

Both of these are wrong, I believe, because the string content is not part of the program indentation.

python -t does not warn about this either.