Remove unrequired encoding header

[Alisa-lisa]

This is not required anymore for Python 3.

[pganssle]

This library still supports Python 2 for at least a little while, though, I think.

Also, while not required I don't think it hurts to explicitly declare the encoding of the file even if it's the same as the default.

[kngwyu]

Let's leave it until python2 support ends :)

[konstin]

Yep, but I think we can keep this open until then.

[milesgranger]

I'd just add that while it's the default (utf-8) for Python3, so far as Python3 is concerned, it makes no difference, but IDEs will can still make use of this declaration.