Installing both 'libapache2-mod-wsgi-py3' and 'libapache2-mod-python' makes apache
unable to respond.
error.log contains something like this (one new line per second):
 [Mon Jan 16 19:17:32 2012] [notice] child pid 6281 exit signal Segmentation fault (11)
 [Mon Jan 16 19:17:32 2012] [notice] child pid 6282 exit signal Segmentation fault (11)
 [Mon Jan 16 19:17:32 2012] [notice] child pid 6283 exit signal Segmentation fault (11)
 [Mon Jan 16 19:17:33 2012] [notice] child pid 6288 exit signal Segmentation fault (11)
 [Mon Jan 16 19:17:34 2012] [notice] child pid 6289 exit signal Segmentation fault (11)
 [Mon Jan 16 19:17:34 2012] [notice] child pid 6290 exit signal Segmentation fault (11)
 [Mon Jan 16 19:17:35 2012] [notice] child pid 6297 exit signal Segmentation fault (11)
 [Mon Jan 16 19:17:35 2012] [notice] child pid 6298 exit signal Segmentation fault (11)
 [Mon Jan 16 19:17:35 2012] [notice] child pid 6299 exit signal Segmentation fault (11)
 [Mon Jan 16 19:17:36 2012] [notice] child pid 6300 exit signal Segmentation fault (11)
 [Mon Jan 16 19:17:36 2012] [notice] child pid 6301 exit signal Segmentation fault (11)
 ...

The symptoms are similar to those described in bug #558608 but I'm not sure the cause is the same.

Resolving the problem was easy for me since I didn't need libapache2-mod-python anymore
so I purged it and everything went fine then.

Jessie version shows a better error message, which is a real improvement:

[wsgi:crit] [pid 17100] mod_wsgi (pid=17100): The mod_python module can
not be used on conjunction with mod_wsgi 4.0+. Remove the mod_python
module from the Apache configuration.

However, a "Conflicts: libapache2-mod-python" seems a good idea to me.