symlink python3 to python (#7103)

* symlink python3 to python

* otherwise not all autotools are smart enough

* Update package.py

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

@@ -203,6 +203,12 @@ def post_install(self):
                 os.symlink(os.path.join(src, f),
                            os.path.join(dst, f))
 
+        if spec.satisfies('@3:'):
+            os.symlink(os.path.join(prefix.bin, 'python3'),
+                       os.path.join(prefix.bin, 'python'))
+            os.symlink(os.path.join(prefix.bin, 'python3-config'),
+                       os.path.join(prefix.bin, 'python-config'))
+
     # TODO: Once better testing support is integrated, add the following tests
     # https://wiki.python.org/moin/TkInter
     #