Simplify UTF8 StrPtr usage (#2374)

* Use non-BOM encodings

* Copy potential BOM to the output of PyString_FromString

The documentation of the used `PyUnicode_DecodeUTF16` states that not
passing `*byteorder` or passing a 0 results in the first two bytes, if
they are the BOM (U+FEFF, zero-width no-break space), to be interpreted
and skipped, which is incorrect when we convert a known "non BOM"
string, which all strings from C# are.

* Default to UTF8 for StrPtr