REF: Add keywordonly module to deprecation cycle

Marked for removal in version 5.0.0

Author: rmarkello

nibabel/keywordonly.py

@@ -2,6 +2,13 @@
 """
 
 from functools import wraps
+import warnings
+
+warnings.warn("We will remove this module from nibabel 5.0. "
+              "Please use the built-in Python `*` argument to ensure "
+              "keyword-only parameters (see PEP 3102).",
+              DeprecationWarning,
+              stacklevel=2)
 
 
 def kw_only_func(n):

nibabel/tests/test_removalschedule.py

@@ -2,6 +2,7 @@
 from ..testing import assert_raises, assert_false
 
 MODULE_SCHEDULE = [
+    ('5.0.0', ['nibabel.keywordonly']),
     ('4.0.0', ['nibabel.trackvis']),
     ('3.0.0', ['nibabel.minc', 'nibabel.checkwarns']),
     # Verify that the test will be quiet if the schedule outlives the modules