Fix crash if byte-compiled level 2

[JGoutin]

Fix crash if byte-compiled level 2 (Python -OO command line argument)

[tacaswell]

Is this really the only place that breaks? We have some pretty extensive docstring substitution/rewriting code.

[JGoutin]

After fixing this, I get no more crash on importing Matplotlib. But, I don't use all Matplotlib possibilities in my software and don't import everything.

The bug come only when __doc__ ( = None if byte compiled level 2) is used with a str function or operator.

[jenshnielsen]

I guess if we want to ensure that this works one of the jobs on travis should run the tests with -OO ?

[tacaswell]

Indeed, I nominate the 3.4 run (2.7 has the old deps, 3.5 has pandas, and the nightly is an optional fail).

[JGoutin]

Tried to update .travis.yml to include test with -OO. But failed before test.

[tacaswell]

This seems to need a rebase?

[JGoutin]

Go to #6729