Fix unicode formatting.

[pkaleta]

On Python 2.x, scheduler.Task.pretty_id methods fails with the following error when a parameter's value contains a unicode code point:

Traceback (most recent call last):
  File "/Users/piotr/counsyl/luigi/test/scheduler_test.py", line 34, in test_pretty_id_unicode
    task.pretty_id
  File "/Users/piotr/counsyl/luigi/luigi/scheduler.py", line 338, in pretty_id
    param_str = ', '.join('{}={}'.format(key, value) for key, value in sorted(self.params.items()))
  File "/Users/piotr/counsyl/luigi/luigi/scheduler.py", line 338, in <genexpr>
    param_str = ', '.join('{}={}'.format(key, value) for key, value in sorted(self.params.items()))
UnicodeEncodeError: 'ascii' codec can't encode character u'\u2192' in position 0: ordinal not in range(128)

Description

Do not attempt to convert unicode to str, but return unicode object instead.

Have you tested this? If so, how?

I have included unit tests. I've also ran my jobs with this code and it works for me.

[dlstadther]

LGTM

[pkaleta]

Thanks @dlstadther. Can one of the maintainers merge this? I don't have rights to do that.

[pkaleta]

friendly ping on this one

[pkaleta]

Thanks @dlstadther!

[Tarrasch]

Thanks @pkaleta!