Lowpass filter with a time-varying cutoff frequency, with Python

How to apply a lowpass filter, with cutoff frequency varying linearly (or with a more general curve than linear) from e.g. 10000hz to 200hz along time, with numpy/scipy and possibly no other library?

Example:

• at 00:00,000, lowpass cutoff = 10000hz
• at 00:05,000, lowpass cutoff = 5000hz
• at 00:09,000, lowpass cutoff = 1000hz
• then cutoff stays at 1000hz during 10 seconds, then cutoff decreases down to 200hz

Here is how to do a simple 100hz lowpass:

from scipy.io import wavfile
import numpy as np
from scipy.signal import butter, lfilter

sr, x = wavfile.read('test.wav')
b, a = butter(2, 100.0 / sr, btype='low')  # Butterworth
y = lfilter(b, a, x)
wavfile.write('out.wav', sr, np.asarray(y, dtype=np.int16))

but how to make the cutoff vary?

Note: I've already read Applying time-variant filter in Python but the answer is quite complex (and it applies to many kinds of filter in general).

from Lowpass filter with a time-varying cutoff frequency, with Python