Allow ostream-redirection of more than 1024 characters

[Quincunx271]

Fixes #1478

In short, detail::pythonbuf::overflow() had its branch for the return statement backwards, since bool(sync()) is true if and only if sync() fails; sync() returns 0 on success. This fixes it by using sync() == 0 as the condition.

[Quincunx271]

The CI errors seem to be a warning about using a deprecated numpy type (matrix), which has nothing to do with this change. All non-numpy tests seem to be passing.

[wjakob]

There is something I don't understand about the original implementation, which is somewhat related to the patch here. It looks like sync() can't return any value other than zero -- so what's the purpose of the ternary expression?

[Quincunx271]

If someone wrote a class which inherits from pythonbuf, sync() could return a non-zero value.

[wjakob]

Ah, gotcha -- it's a virtual function in the C++ standard. This looks good to me then.