Fix missing warnings import in phase_cross_correlation (#7175)

* Add missing warnings import

Hups, 05e84c2 removed the import of warnings thus breaking the merge
result after merging #7147!

* Test no warning is raised in phase_cross_correlation

when disambiguate=True. See
#7146

skimage/registration/_phase_cross_correlation.py

@@ -4,6 +4,7 @@
 """
 
 import itertools
+import warnings
 
 import numpy as np
 from scipy.fft import fftn, ifftn, fftfreq

skimage/registration/tests/test_phase_cross_correlation.py

@@ -1,4 +1,5 @@
 import itertools
+import warnings
 
 import numpy as np
 import pytest
@@ -206,6 +207,17 @@ def test_disambiguate_2d(shift0, shift1):
     np.testing.assert_equal(shift, computed_shift)
 
 
+def test_invalid_value_in_division_warnings():
+    """Regression test for https://github.com/scikit-image/scikit-image/issues/7146."""
+    im1 = np.zeros((100, 100))
+    im1[50, 50] = 1
+    im2 = np.zeros((100, 100))
+    im2[60, 60] = 1
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        phase_cross_correlation(im1, im2, disambiguate=True)
+
+
 @pytest.mark.parametrize('disambiguate', [True, False])
 def test_disambiguate_zero_shift(disambiguate):
     """When the shift is 0, disambiguation becomes degenerate.