Add timespec argument to to_iso8601_string() function

[Itay4]

Fixes #347

[Itay4]

currently python 2.7 build fails, as datetime for it does not support timespec argument
how would you like me to handle that?
should i check the version and split code accordingly ?

[joaonc]

I would love to see this PR merged, however doing it would mean that the behavior would be different for Python 3.6+ and lower versions (note: not just python 2.7, as this argument was introduced in 3.6).

On the good side, the 3.5- users wouldn't see any change, they just wouldn't be able to use the new feature.

The options are:

1. Leave as-is and the new parameter would only be available in 3.6+ (easiest).
   This would probably mean check for version and split code.
2. Implement the whole timespec functionality in pendulum and make it available for everyone.
   This would open a whole new can of worms, probably (haven't checked the implementation in the datetime package.

My suggestion is going with option 1 and make a note of it in the documentation.
In 3.5-, take the argument but do nothing with it.

[Itay4]

Can someone assist and explain why Travis build fails ?

[joaonc]

@Itay4 looks like it was the linting step that failed, meaning the warning that black is issuing here is causing the build to fail. Something with code style, is my guess.