Remove the [test] extras_requires.

Test dependencies, along with how tests are run, are now specified in
tox.ini.

Author: dairiki

setup.cfg

@@ -52,15 +52,6 @@ setup_requires =
 [options.extras_require]
 ipython =
     ipython
-# XXX: the [test] extras is deprecated
-# Ref: https://github.com/lektor/lektor/pull/931#issuecomment-911879538
-test =
-    pre-commit
-    pylint==2.7.0
-    pytest
-    pytest-cov
-    pytest-mock
-    pytest-click
 
 [options.entry_points]
 console_scripts =