disable xapian for python 3.11-dev

do not install xapian for python-3.11-dev tests. Build is failing and
I am not sure why.

/home/travis/virtualenv/python3.11-dev/bin/python3 -c 'import os;os.chdir("xapian");import _xapian'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ModuleNotFoundError: No module named '_xapian'

Author: rouilj

.travis.yml

@@ -68,7 +68,7 @@ before_install:
   - if [[ $TRAVIS_PYTHON_VERSION == "3."* ]]; then ./configure --prefix=$VIRTUAL_ENV --with-python3; fi
   - if [[ $TRAVIS_PYTHON_VERSION == "nightly" ]]; then ./configure --prefix=$VIRTUAL_ENV --with-python3; fi
   - if [[ $TRAVIS_PYTHON_VERSION == "pypy3" ]]; then ./configure --prefix=$VIRTUAL_ENV --with-python3; fi
-  - make && make install
+  - if [[ $TRAVIS_PYTHON_VERSION != "3.11-dev" ]]; then make && make install; fi
 
   - PATH=$VIRTUAL_ENV/bin:$PATH