"U" mode is no longer accepted from Python 3.9

[tirkarthi]

mccabe/mccabe.py

Line 294 in e92e9e7

with open(module_path, "rU") as mod:

mccabe/mccabe.py
294:    with open(module_path, "rU") as mod:
301:        with open(filename, 'rU') as f:

CPython PR : python/cpython#16959