Use more modern string formatting internally within param

[ceball]

We've been using printf-style formatting (https://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting), but python now offers nicer ways to format strings.

Note that we can't use the following, because it's not available in 2:

Changed in version 3.1: The positional argument specifiers can be omitted, so '{} {}' is equivalent to '{0} {1}'

[kcpevey]

Is this referring to the internals of param? If so, yes, it would be great to update that.