Pin Aer to known good version (#10270) (#10272)

* Pin Aer to known good version

The release of Aer 0.12.1 included a bug in the results output that made
the general `Result` object unpickleable.  Several places in our tests
assume that these objects should be able to be pickled to be sent across
a process boundary, or such like.

* Make GitHub Actions respect constraints

(cherry picked from commit 676d90c)

Co-authored-by: Jake Lishman <jake.lishman@ibm.com>

.github/workflows/coverage.yml

@@ -38,10 +38,10 @@ jobs:
       # Modern pip (23.1+) can error out if it doesn't have `wheel` and we ask for one
       # of these legacy packages.
       - name: Ensure basic build requirements
-        run: python -m pip install --upgrade pip setuptools wheel
+        run: python -m pip install -c constraints.txt --upgrade pip setuptools wheel
 
       - name: Build and install qiskit-terra
-        run: python -m pip install -e .
+        run: python -m pip install -c constraints.txt -e .
         env:
           CARGO_INCREMENTAL: 0
           RUSTFLAGS: "-Cinstrument-coverage"
@@ -51,7 +51,7 @@ jobs:
       - name: Generate unittest coverage report
         run: |
           set -e
-          python -m pip install -r requirements-dev.txt qiskit-aer
+          python -m pip install -c constraints.txt -r requirements-dev.txt qiskit-aer
           stestr run
           # We set the --source-dir to '.' because we want all paths to appear relative to the repo
           # root (we need to combine them with the Python ones), but we only care about `grcov`

constraints.txt

@@ -1,3 +1,7 @@
 # jsonschema pinning needed due nbformat==5.1.3 using deprecated behaviour in
 # 4.0+. The pin can be removed after nbformat is updated.
 jsonschema==3.2.0
+
+# Aer 0.12.1 has a bad value in the results output that prevents pickling.
+# Remove pin once https://github.com/Qiskit/qiskit-aer/pull/1845 is released.
+qiskit-aer==0.12.0