update python versions

CONTRIBUTING.rst

@@ -102,7 +102,7 @@ Before you submit a pull request, check that it meets these guidelines:
 2. If the pull request adds functionality, the docs should be updated. Put
    your new functionality into a function with a docstring, and add the
    feature to the list in README.rst.
-3. The pull request should work for Python 2.7, 3.4, 3.5 and 3.6, and for PyPy. Check
+3. The pull request should work for Python 3.8, 3.9, 3.10, and 3.11 as well as for PyPy. Check
    https://travis-ci.org/bihealth/altamisa/pull_requests
    and make sure that the tests pass for all supported Python versions.