Use unittest.SkipTest (again?)

While I can believe that the standard exception might not have worked
with some ancient version of pytest, it certainly works just fine now.
This saves people from importing nose which kills pytest
via DeprecationWarnings these days.

Closes: #316
Pull-request: https://github.com/sqlalchemy/mako/pull/316
Pull-request-sha: 90dfcfc3d0351ab9bf8125e8c2adb45c57059a6d

Change-Id: Ie816acd325c6783e08801bc83de37dbe6b4581e5
2 files changed