Add support for bytes literals

[jaraco]

In #710, I described a need for better unicode literals support in doctests.

I now see that I have the same need for bytes literals.

Perhaps @nicoddemus would be interested in replicating the behavior from #938 to also add an ALLOW_BYTES doctests option flag.

[nicoddemus]

Sure, I'll give it a go. 😄

[RonnyPfannschmidt]

Merged