tests: address PytestDeprecationWarning for @pytest.yield_fixture

Adhere to the warning:

  kallithea/tests/conftest.py:207: PytestDeprecationWarning: @pytest.yield_fixture is deprecated.
  Use @pytest.fixture instead; they are the same.
    @pytest.yield_fixture(scope="session")

kallithea/tests/conftest.py

@@ -204,7 +204,7 b' class MyWSGIServer(WSGIServer):'
         return '%s://%s%s:%s/%s' % (proto, auth, host, port, repo_name)
 
 
[e-mail]_fixture(scope="session")
[e-mail](scope="session")
 def webserver():
     """Start web server while tests are running.
     Useful for debugging and necessary for vcs operation tests."""