root: Configure ROOT correctly for python3 (#14225)

* Configure ROOT correctly for python3

* Record Python version conflict

* Code editing on a phone sucks

var/spack/repos/builtin/packages/root/package.py

@@ -241,6 +241,9 @@ class Root(CMakePackage):
     # GCC 9.2.1, which we can safely extrapolate to the GCC 9 series.
     conflicts('%gcc@9.0.0:', when='@:6.11.99')
 
+    # ROOT <6.14 was incompatible with Python 3.7+
+    conflicts('python@3.7:', when='@:6.13.99 +python')
+
     # See README.md
     conflicts('+http',
               msg='HTTP server currently unsupported due to dependency issues')
@@ -358,8 +361,7 @@ def cmake_args(self):
                 ['pgsql', 'postgres'],
                 ['pythia6'],
                 ['pythia8', False],
-                ['python', self.spec.satisfies('+python ^python@2.7:2.99.99')],
-                ['python3', self.spec.satisfies('+python ^python@3.0:')],
+                ['python', self.spec.satisfies('+python')],
                 ['qt', 'qt4'],  # See conflicts
                 ['qtgsi', 'qt4'],  # See conflicts
                 ['r', 'R'],