Add extra version of py-pyqt5 (#15071)

var/spack/repos/builtin/packages/py-pyqt5/package.py

@@ -27,6 +27,7 @@ class PyPyqt5(SIPPackage):
     ]
 
     version('5.13.0', sha256='0cdbffe5135926527b61cc3692dd301cd0328dd87eeaf1313e610787c46faff9')
+    version('5.12.3', sha256='0db0fa37debab147450f9e052286f7a530404e2aaddc438e97a7dcdf56292110')
 
     variant('qsci', default=False, description='Build with QScintilla python bindings')