conbytes

[samuelcolvin]

As per #311 (comment) we should implement conbytes and suport it in Schema.

Exactly the same as constr but for bytes.

cc @tiangolo

[tiangolo]

@samuelcolvin Do you want me to implement it as part of #311 ? Or should it be a posterior PR?

[samuelcolvin]

Definitely new PR. If you'd like to do it that would be great. Otherwise I can.