Fix compatibility with the latest release of Sphinx

`l_` from sphinx.locale has been deprecated for a long time.
`_` is the new name for the same function but it seems that the
imports there are useless.

https://github.com/sphinx-doc/sphinx/commit/8d653a406dc0dc6c2632176ab4757ca15474b10f

docs/sphinxext/configtraits.py

@@ -1,17 +1,16 b''
 """Directives and roles for documenting traitlets config options.
 
 ::
 
     .. configtrait:: Application.log_datefmt
 
         Description goes here.
 
     Cross reference like this: :configtrait:`Application.log_datefmt`.
 """
-from sphinx.locale import l_
-from sphinx.util.docfields import Field
+
 
 def setup(app):
     app.add_object_type('configtrait', 'configtrait', objname='Config option')
     metadata = {'parallel_read_safe': True, 'parallel_write_safe': True}
     return metadata