Introduce version info as a tuple of int

[kitchoi]

ETS packages have __version__ as a string, e.g. "4.5.0". That is fine and is an expected format.

But sometimes we want to use the version information for things like skipping a test for a specific version range (e.g. https://github.com/enthought/pyface/pull/656), then we have to resort to parsing the version string. It is not hard to do, but it would have been easier if a version_info or __version_info__ as a tuple of int is provided by the package already along with __version__.

e.g.

__version__ = "6.1.0"
__version_info__ = (6, 1, 0)

A similar topic was discussed a long time ago: https://github.com/enthought/distributed-array-protocol/issues/16