Try importing PySide, if we can't find sip.

If no QT_API is specified, we use the value in rcParams and the
default value for this is PyQt.  If a user has PySide installed,
trying to use the qt backend fails, unless QT_API is specified or the
config file is modified.  This commit, adds code to try and import
PySide, if an import of PyQt fails.
[DOC] Add documentation for the QT_API environment variable.