Pin symengine in backwards compatibility tests (#13885)

* Add symengine constraint in qpy backwards compat tests

* Add comments

* Only pin upper limit

* Update comment

(cherry picked from commit 8c06fd2)

Author: ElePT

requirements.txt

@@ -7,6 +7,7 @@ python-dateutil>=2.8.0
 stevedore>=3.0.0
 typing-extensions
 
-# If updating the version range here, consider updating 'test/qpy_compat/run_tests.sh' to update the
-# list of symengine dependencies used in the cross-version tests.
+# If updating the version range here, consider updating the 
+# list of symengine dependencies used in the cross-version tests
+# in 'test/qpy_compat/run_tests.sh' and 'test/qpy_compat/qpy_test_constraints.txt'
 symengine>=0.11,<0.14

test/qpy_compat/qpy_test_constraints.txt

@@ -1,2 +1,7 @@
 numpy===1.24.4
 scipy===1.10.1
+
+# This is a loose constraint because we want to test different versions,
+# as defined in 'test/qpy_compat/run_tests.sh', but any symengine version 
+# above (and including) 0.14 will be incompatible with qpy.
+symengine<0.14

test/qpy_compat/run_tests.sh

@@ -57,6 +57,8 @@ popd
 # This will likely duplicate the base dev-compatibility test, but the tests are fairly fast, and
 # it's better safe than sorry with the serialisation tests.
 
+# Note that the constraint in the range of symengine versions is logically duplicated 
+# in `qpy_test_constraints.txt`
 symengine_versions=(
     '>=0.11,<0.12'
     '>=0.13,<0.14'