cat.py: fix Python deprecation warning

cat.py: fix Python deprecation warning

This commit fixes the following warning:

  DeprecationWarning: 'U' mode is deprecated

Removing 'U' has no effect with Python 3 [1]:

  There is an additional mode character permitted, 'U', which no
  longer has any effect, and is considered deprecated. It previously
  enabled universal newlines in text mode, which became the default
  behaviour in Python 3.0.

[1] https://docs.python.org/3/library/functions.html?highlight=open#open