wxPython demo and installed wxPython version

[Karsten Hilbert]

When running the wxPython 3.0 demo while wxPython 2 is
installed a warning is shown that some incompatibilities may
show up. After that the demo throws an exception due to
app introspection initiation.

Might it be possible, however, to make use of wxversion
similar to this pseudocode:

try:
import wxversion
try:
select_required_version_of_wx()
except wxversion.VersionError:
raise ImportError
except ImportError:
if not required_wx_version_active():
show_warning()
startup()

?

This change would make it possible to properly run either of
the wxp2 or wxp3 demo on a machine having both wxp2 and wxp3
installed.

Thanks,
Karsten