The output from pygments.lexers.get_all_lexers() contains 'juttle'
twice in the aliases section for the Juttle lexer entry.

This could be reproduced using:

>>> from pygments.lexers import get_all_lexers
>>> lexers = get_all_lexers()
>>> {alias[0]: alias[1] for alias in lexers}.get('Juttle')
('juttle', 'juttle')

This patch fixes the duplicate entry and generates the associated
_mapping.py file.

Fixes: pygments#1604