solving make_spectrogram bug

Bug Description: np.ndarray does not accept float as values for indices [ step = seg_length // 2 ] thus the make_spectrogram function doesn't work.
Solution: solving this issue by ensuring step is an integer [ step = int(seg_length // 2) ] thereby the values of i and j are also integers.

code/thinkdsp.py

@@ -934,7 +934,7 @@ def make_spectrogram(self, seg_length, win_flag=True):
         if win_flag:
             window = np.hamming(seg_length)
         i, j = 0, seg_length
-        step = seg_length // 2
+        step = int(seg_length // 2)
 
         # map from time to Spectrum
         spec_map = {}