graphviz: add python dependency to fix installation (#24852)

The bootstrap script in the autoreconf procedure calls the
gen_version.py script which requires python 3.6 to process f-strings.

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

@@ -116,6 +116,8 @@ class Graphviz(AutotoolsPackage):
     depends_on('sed', type='build')
     depends_on('libtool', type='build')
     depends_on('pkgconfig', type='build')
+    # to process f-strings used in gen_version.py
+    depends_on('python@3.6:', when='@2.47:', type='build')
 
     conflicts('~doc',
               when='@:2.45',