Make string conversion stricter (#695)

* Make string conversion stricter

The string conversion logic added in PR #624 for all std::basic_strings
was derived from the old std::wstring logic, but that was underused and
turns out to have had a bug in accepting almost anything convertible to
unicode, while the previous std::string logic was much stricter.  This
restores the previous std::string logic by only allowing actual unicode
or string types.

Fixes #685.

* Added missing 'requires numpy' decorator

(I forgot that the change to a global decorator here is in the
not-yet-merged Eigen PR)

include/pybind11/cast.h

@@ -657,6 +657,8 @@ struct type_caster<std::basic_string<CharT, Traits, Allocator>, enable_if_t<is_s
             return false;
             // The below is a guaranteed failure in Python 3 when PyUnicode_Check returns false
 #else
+            if (!PYBIND11_BYTES_CHECK(load_src.ptr()))
+                return false;
             temp = reinterpret_steal<object>(PyUnicode_FromObject(load_src.ptr()));
             if (!temp) { PyErr_Clear(); return false; }
             load_src = temp;