Improve spack checksum formatting (#13666)

* Improve spack checksum formatting

* Add colon

* Fix Python 2 bug

lib/spack/spack/cmd/checksum.py

@@ -61,3 +61,4 @@ def checksum(parser, args):
 
     print()
     print(version_lines)
+    print()

lib/spack/spack/stage.py

@@ -3,6 +3,8 @@
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
+from __future__ import print_function
+
 import os
 import stat
 import sys
@@ -760,7 +762,7 @@ def get_checksums_for_versions(
             *spack.cmd.elide_list(
                 ["{0:{1}}  {2}".format(str(v), max_len, url_dict[v])
                  for v in sorted_versions]))
-    tty.msg('')
+    print()
 
     archives_to_fetch = tty.get_number(
         "How many would you like to checksum?", default=1, abort='q')
@@ -807,7 +809,7 @@ def get_checksums_for_versions(
     ])
 
     num_hash = len(version_hashes)
-    tty.msg("Checksummed {0} version{1} of {2}".format(
+    tty.msg("Checksummed {0} version{1} of {2}:".format(
         num_hash, '' if num_hash == 1 else 's', name))
 
     return version_lines