Add a non-const overload of bytearray::data

[tjstum]

Unlike bytes, a bytearray is mutable. You are allowed to write to it, and the underlying API function returns a char*

[tjstum]

This seemed simple enough to skip a test and changelog entry, but I'm happy to add those if that's desired. Thanks!

[Hnasar]

LGTM

[wjakob]

In this case, I would expect to have a const and a non-const version of the cast operator, with only the latter having a non-const return value.

[tjstum]

Thanks for the feedback! Just to be clear, since these are named methods, not operators, are you saying that you'd prefer if this adds c_str_mutable() and data_mutable() functions to the bytearray class?

[oremanj]

I imagine the desire was for overloading on constness:

char* data() { return …; }
const char* data() const { return …; }

I would personally keep the c_str name const-only, but it would also be possible to do the same with it.

[wjakob]

Thanks!