Add __format__ support to version for fancy formatting. (#3996)

- add Version.__format__ to support new-style formatting.
- Python3 doesn't handle this well -- it delegates to
  object.__format__(), which raises an error for fancy format strings.
- not sure why it doesn't call str(self).__format__ instead, but that's
  hwo things are.

lib/spack/spack/version.py

@@ -226,6 +226,9 @@ def __repr__(self):
     def __str__(self):
         return self.string
 
+    def __format__(self, format_spec):
+        return self.string.format(format_spec)
+
     @property
     def concrete(self):
         return self