The gvalue code had special code for handling G_TYPE_CHAR/UCHAR
allowing as input roughly what u/int8 allows plus unicode.

Since GI maps g(u)char to u/int8 it's a good idea that both support
the same input ranges for numbers/bytes so that there is no difference
between gtype/GI.

pygi_gschar_from_py is a superset of gint8 + unicode.
pygi_guchar_from_py is a superset of guint8 + unicode.