int()
is the Python standard built-in function to convert a string into an integer value. You call it with a string containing a number as the argument, and it returns the number converted to an integer:
>>> int("1") + 1
2
If you know the structure of your list, T1 (that it simply contains lists, only one level), you could do this in Python 3:
T2 = [list(map(int, x)) for x in T1]
In Python 2:
T2 = [map(int, x) for x in T1]