input:
[(4,8),(6,10),(11,12),(15,20),(20,25)]
output:
[(4, 12), (15, 25)]
input:
([(4,8),(6,10),(11,12),(15,20)])
output:
[(4, 12), (15, 20)]

import numpy as np

def simplify_intervals(intervals):
    # Make intervals into an array
    intervals = np.asarray(intervals)
    # Make array for zero to the greatest interval end
    r = np.arange(intervals[:, 1].max() + 1)[:, np.newaxis]
    # Check what elements of the array are within each interval
    m = (r >= intervals[:, 0]) & (r <= intervals[:, 1])
    # Collapse belonging test for each value
    ind = m.any(1).astype(np.int8)
    # Find where the belonging test changes
    d = np.diff(np.concatenate([[0], ind, [0]]))
    # Find interval bounds
    start = np.where(d > 0)[0]
    end = np.where(d < 0)[0] - 1
    # Make final intervals array
    return np.stack([start, end], axis=1)

print(simplify_intervals([(4, 8), (6, 10), (11, 12), (15, 20), (20, 25)]))
# [[ 4 12]
#  [15 25]]
print(simplify_intervals(([(4,8),(6,10),(11,12),(15,20)])))
# [[ 4 12]
#  [15 20]]