python: Stop using the string module

python: Stop using the string module

Most functions in the builtin string module also exist as methods of
string objects.

Since the functions were removed from the string module in Python 3,
using the instance methods directly makes the code compatible with both
Python 2 and Python 3.

Signed-off-by: Mathieu Bridon <bochecha@daitauha.fr>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>