Pin z3-solver on macOS VMs (Qiskit#11408)

We currently use macOS 11.7 images in our CI and CPython 3.8 for the
all-optionals test run.  `z3-solver` recently released 4.12.3.0, which
upped the macOS platform version from 10.16 to 11.7.  This in theory
should be fine for our VM image, but the pre-built version of CPython
3.8 we have access to was built for an older macOS, so does not match a
`macos_11_7_x86_64` platform tag, and forces us to build Z3 from source,
often timing out the job.

This should have no effect on user machines, which will be typically be
using newer versions of Python, or will be able to install from source
if required.

This CI-only constraint can be relaxed when the version of CPython we
use on the macOS VMs supports the 11.7 macOS API version.

constraints.txt

@@ -6,3 +6,10 @@ jsonschema==3.2.0
 # eigensystem code for one of the test cases.  See
 # https://github.com/Qiskit/qiskit-terra/issues/10345 for current details.
 scipy<1.11; python_version<'3.12'
+
+# z3-solver from 4.12.3 onwards upped the minimum macOS API version for its
+# wheels to 11.7. The Azure VM images contain pre-built CPythons, of which at
+# least CPython 3.8 was compiled for an older macOS, so does not match a
+# `macos_11_7` platform tag.  This should be purely a CI artefact, and not
+# affect local usage.
+z3-solver==4.12.2.0; platform_system=="Darwin"