gh-37043: src/sage/matrix/matrix_double_dense.pyx: increase an FP tolerance
I recently hit,
```
File "src/sage/matrix/matrix_double_dense.pyx", line 1629, in
sage.matrix.matrix_double_dense.Matrix_double_dense.right_eigenvectors
Failed example:
spectrum[3] # tol 1e-13
Expected:
(-1.0, [(1.0, -0.5, 2.0, 0.5)], 1)
Got:
(-1.0000000000000828,
[(1.0, -0.4999999999999238, 1.9999999999997218,
0.4999999999999161)],
1)
Tolerance exceeded in 3 of 6:
-0.5 vs -0.4999999999999238, tolerance 2e-13 > 1e-13
2.0 vs 1.9999999999997218, tolerance 2e-13 > 1e-13
0.5 vs 0.4999999999999161, tolerance 2e-13 > 1e-13
```
This commit changes that tolerance to 1e-12 and makes the failure go
away.
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URL: https://github.com/sagemath/sage/pull/37043
Reported by: Michael Orlitzky
Reviewer(s): Matthias Köppe
