Fix py-yt for Python 2 by requesting older py-ipython version (#11416)

* Fix py-yt for Python 2 by requesting older py-ipython version.
* Add conflicts statement to py-ipython because version 7: is not
  compatible with python@2.7

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

@@ -36,3 +36,5 @@ class PyIpython(PythonPackage):
     depends_on('py-appnope', type=('build', 'run'),
                     when=sys.platform == 'darwin' and
                             int(platform.mac_ver()[0].split('.')[1]) >= 9)
+
+    conflicts('^python@2.7:2.8', when='@7.0.0:')

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

@@ -44,6 +44,7 @@ class PyYt(PythonPackage):
     depends_on("py-cython", type=('build', 'run'))
     depends_on("py-h5py", type=('build', 'run'), when="+h5py")
     depends_on("py-ipython", type=('build', 'run'))
+    depends_on("py-ipython@:6.99", type=('build', 'run'), when="^python@:2.99")
     depends_on("py-matplotlib", type=('build', 'run'))
     depends_on("py-numpy", type=('build', 'run'))
     depends_on("py-scipy", type=('build', 'run'), when="+scipy")