update xrange() to range()

update xrange() to range() as Python 2 has been deprecate, more info:
1.  Python 2 has been no longer supported officially since January 1, 2020.  Check  https://www.python.org/doc/sunset-python-2/
2.  xrange() was renamed to range() in Python 3. Check  https://stackoverflow.com/questions/17192158/nameerror-global-name-xrange-is-not-defined-in-python-3/17192181#17192181

update xrange() to range()

Update py_fourier_transform.markdown

update xrange() to range()

doc/py_tutorials/py_imgproc/py_thresholding/py_thresholding.markdown

@@ -47,7 +47,7 @@ ret,thresh5 = cv.threshold(img,127,255,cv.THRESH_TOZERO_INV)
 titles = ['Original Image','BINARY','BINARY_INV','TRUNC','TOZERO','TOZERO_INV']
 images = [img, thresh1, thresh2, thresh3, thresh4, thresh5]
 
-for i in xrange(6):
+for i in range(6):
     plt.subplot(2,3,i+1),plt.imshow(images[i],'gray',vmin=0,vmax=255)
     plt.title(titles[i])
     plt.xticks([]),plt.yticks([])
@@ -98,7 +98,7 @@ titles = ['Original Image', 'Global Thresholding (v = 127)',
             'Adaptive Mean Thresholding', 'Adaptive Gaussian Thresholding']
 images = [img, th1, th2, th3]
 
-for i in xrange(4):
+for i in range(4):
     plt.subplot(2,2,i+1),plt.imshow(images[i],'gray')
     plt.title(titles[i])
     plt.xticks([]),plt.yticks([])
@@ -153,7 +153,7 @@ titles = ['Original Noisy Image','Histogram','Global Thresholding (v=127)',
           'Original Noisy Image','Histogram',"Otsu's Thresholding",
           'Gaussian filtered Image','Histogram',"Otsu's Thresholding"]
 
-for i in xrange(3):
+for i in range(3):
     plt.subplot(3,3,i*3+1),plt.imshow(images[i*3],'gray')
     plt.title(titles[i*3]), plt.xticks([]), plt.yticks([])
     plt.subplot(3,3,i*3+2),plt.hist(images[i*3].ravel(),256)
@@ -196,7 +196,7 @@ bins = np.arange(256)
 fn_min = np.inf
 thresh = -1
 
-for i in xrange(1,256):
+for i in range(1,256):
     p1,p2 = np.hsplit(hist_norm,[i]) # probabilities
     q1,q2 = Q[i],Q[255]-Q[i] # cum sum of classes
     if q1 < 1.e-6 or q2 < 1.e-6: