Fix warning on __qiskit_version__ (Qiskit#10686)

* Fix warning on `__qiskit_version__`

The warning emitted by the `QiskitVersion` class that backs the
`__qiskit_version__` variable was previously set to warn on
instantiation, not usage.  This caused the warning to trigger during
library import, rather than at the site of usage.

* Add warning assertion to test

qiskit/version.py

@@ -93,21 +93,23 @@ class QiskitVersion(Mapping):
     __slots__ = ["_version_dict", "_loaded"]
 
     def __init__(self):
+        self._version_dict = {
+            "qiskit-terra": __version__,
+            "qiskit": None,
+        }
+        self._loaded = False
+
+    def _load_versions(self):
         warnings.warn(
             "qiskit.__qiskit_version__ is deprecated since "
             "Qiskit Terra 0.25.0, and will be removed 3 months or more later. "
-            "Instead, you should use qiskit.__version__. The other packages listed in "
+            "Instead, you should use qiskit.__version__. The other packages listed in the"
             "former qiskit.__qiskit_version__ have their own __version__ module level dunder, "
             "as standard in PEP 8.",
             category=DeprecationWarning,
+            stacklevel=3,
         )
-        self._version_dict = {
-            "qiskit-terra": __version__,
-            "qiskit": None,
-        }
-        self._loaded = False
 
-    def _load_versions(self):
         from importlib.metadata import version
 
         try:

test/python/test_version.py

@@ -22,4 +22,5 @@ class TestVersion(QiskitTestCase):
 
     def test_qiskit_version(self):
         """Test qiskit-version sets the correct version for terra."""
-        self.assertEqual(__version__, __qiskit_version__["qiskit-terra"])
+        with self.assertWarnsRegex(DeprecationWarning, "__qiskit_version__"):
+            self.assertEqual(__version__, __qiskit_version__["qiskit-terra"])