Use numpy eigh instead of scipy eigh with driver kwarg

In Qiskit#5251 we updated the LAPACK driver used for the scipy hermitian
optimized eigensovler to use 'evd' instead of the default 'ev'. However,
this implicitly added a scipy minimum version of 1.5.0 because the
driver kwarg was only added to the eigh() function in that release. [1]
In that PR we didn't bump the minimum scipy version to reflect this.
However, scipy 1.5.0 has a version conflict with our downstream users,
mainly qiskit-aqua/qiskit-chemistry which use PySCF as an optional
dependency. PySCF has a capped version of scipy in their requirements [2]
which makes bumping our minimum scipy version intractable currently. To
avoid this issue, this commit switches to use numpy's eigh()
function [3] instead of scipy's. The numpy function uses the evd driver
and was what we originally used in Qiskit#5251 but was switched to use scipy's
eigh() with the driver kwarg to avoid bumping the minimum version of
numpy. However, between bumping the minimum numpy version and bumping
the minimum scipy version, the numpy version is less problematic.

[1] https://docs.scipy.org/doc/scipy/reference/release.1.5.0.html#scipy-linalg-improvements
[2] https://github.com/pyscf/pyscf/blob/v1.7.5/setup.py#L490
[3] https://numpy.org/doc/stable/reference/generated/numpy.linalg.eigh.html

qiskit/quantum_info/synthesis/two_qubit_decompose.py

@@ -169,7 +169,7 @@ def __init__(self, unitary_matrix, eps=1e-15):
         for i in range(100):  # FIXME: this randomized algorithm is horrendous
             state = np.random.default_rng(i)
             M2real = state.normal()*M2.real + state.normal()*M2.imag
-            _, P = la.eigh(M2real, driver='evd')
+            _, P = np.linalg.eigh(M2real, driver='evd')
             D = P.T.dot(M2).dot(P).diagonal()
             if np.allclose(P.dot(np.diag(D)).dot(P.T), M2, rtol=1.0e-13, atol=1.0e-13):
                 break

releasenotes/notes/numpy-bump-b14a4a0ec1c0636f.yaml

@@ -0,0 +1,9 @@
+---
+upgrade:
+  - |
+    The minimum required version of numpy has been increased from 1.17.0 to
+    1.18.0. This was done to leverage the ``numpy.linalg.eigh()`` function [#]_
+    for use in :class:`qiskit.quantum_info.TwoQubitBasisDecomposer` which was
+    needed to provide consistent results.
+
+    .. [#] https://numpy.org/doc/stable/reference/generated/numpy.linalg.eigh.html

requirements.txt

@@ -2,7 +2,7 @@ contextvars>=2.4;python_version<'3.7'
 jsonschema>=2.6
 networkx>=2.2
 retworkx>=0.5.0
-numpy>=1.17
+numpy>=1.18
 ply>=3.10
 psutil>=5
 scipy>=1.4

setup.py

@@ -27,7 +27,7 @@
     "jsonschema>=2.6",
     "networkx>=2.2",
     "retworkx>=0.5.0",
-    "numpy>=1.17",
+    "numpy>=1.18",
     "ply>=3.10",
     "psutil>=5",
     "scipy>=1.4",