Subject: [PATCH] signal: allow floating-point rounding in STFT tests

Keep the STFT tests enabled for x86-64-v3 builds. Allow two float32
epsilons of relative error when comparing inverse-STFT implementations;
float64 and the existing i686 override remain unchanged. Allow one
float64 epsilon of absolute error in all three scaling round trips,
which otherwise require exact zeros (observed residual: 4e-17 for a
signal with amplitude 2).

Upstream issue: https://github.com/scipy/scipy/issues/25488

--- a/scipy/signal/tests/_scipy_spectral_test_shim.py
+++ b/scipy/signal/tests/_scipy_spectral_test_shim.py
@@ -294,7 +294,7 @@
 
     # Adapted tolerances to account for resolution loss:
     atol = np.finfo(x.dtype).resolution*2  # instead of default atol = 0
-    rtol = 1e-7  # default for np.allclose()
+    rtol = max(1e-7, 2 * np.finfo(x.dtype).eps)
 
     # Relax atol on 32-Bit platforms a bit to pass CI tests.
     #  - Not clear why there are discrepancies (in the FFT maybe?)
--- a/scipy/signal/tests/test_spectral.py
+++ b/scipy/signal/tests/test_spectral.py
@@ -2044,7 +2044,7 @@
 
         # Test round trip:
         x1 = istft(Zs, boundary=True, scaling='spectrum')[1]
-        assert_allclose(x1, x)
+        assert_allclose(x1, x, atol=np.finfo(x.dtype).eps)
 
         # For a Hann-windowed 256 sample length FFT, we expect a peak at
         # frequency 64 (since it is 1/4 the length of X) with a height of 1
@@ -2074,7 +2074,7 @@
 
         # Test round trip:
         x1 = istft(Zp, input_onesided=False, boundary=True, scaling='psd')[1]
-        assert_allclose(x1, x)
+        assert_allclose(x1, x, atol=np.finfo(x.dtype).eps)
 
         # The power of the one-sided psd-scaled STFT can be determined
         # analogously (note that the two sides are not of equal shape):
@@ -2094,7 +2094,7 @@
 
         # Test round trip:
         x1 = istft(Zp0, input_onesided=True, boundary=True, scaling='psd')[1]
-        assert_allclose(x1, x)
+        assert_allclose(x1, x, atol=np.finfo(x.dtype).eps)
 
 
 class TestSampledSpectralRepresentations: