Fix SyntaxWarning: "is" with a literal.

[frenzymadness]

When using ipython with Python 3.8.0a4 I discovered this problem in traitlets library.

Reproducer:

Python 3.8.0a4 (default, May 17 2019, 21:21:00) 
[GCC 9.1.1 20190503 (Red Hat 9.1.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import traitlets.config.loader
/home/lbalhar/Software/traitlets/traitlets/config/loader.py:921: SyntaxWarning: "is" with a literal. Did you mean "=="?
  keys = ('-'+key, '--'+key) if len(key) is 1 else ('--'+key, )
/home/lbalhar/Software/traitlets/traitlets/config/loader.py:946: SyntaxWarning: "is" with a literal. Did you mean "=="?
  keys = ('-'+key, '--'+key) if len(key) is 1 else ('--'+key, )
>>>

[frenzymadness]

Travis errors don't seem to be related to these changes.

[rmorshea]

@frenzymadness thanks!