setup: define test dependencies with extras_require

tests_require is only supported by setuptools' test entry point which is
deprecated, extras_require is supported by most tools.
4 jobs for <span class="ref-name">feat/extra_require</span> in 0 seconds
Status Job ID Name Coverage
  External
running SlapOS.Eggs.UnitTest-Jerome.Python2

19487:44:43

passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:12:33

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:11:37

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:13:40