Restrict version-number output of OpenQASM 3 exporter

The exporter previously claimed to simply be `OPENQASM 3;`, though the
OpenQASM project has made it clear that they may include minor versions
of the language in the future.  This updates the exporter to claim to be
version `3.0`, so that it does not falsely imply that it matches syntax
of a later version of the language specification.

qiskit/qasm3/exporter.py

@@ -435,7 +435,7 @@ def _lookup_variable(self, variable) -> ast.Identifier:
 
     def build_header(self):
         """Builds a Header"""
-        version = ast.Version("3")
+        version = ast.Version("3.0")
         includes = self.build_includes()
         return ast.Header(version, includes)
 

releasenotes/notes/qasm3-minor-version-2ae00ba8f72a1a53.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    The OpenQASM 3 exporters :func:`.qasm3.dump` and :func:`~.qasm3.dumps` will now correctly output
+    files claiming to be version ``3.0`` rather than the unqualified ``3``, since the OpenQASM 3
+    project has now standardised on versioning.