To use Python 3.9 you will have to modify the sigil_constants.cpp file that looks like this now, changing the 3.8 to 3.9 in the right places.
Code:
#if __APPLE__
const QString PATH_LIST_DELIM = ":";
const QString PYTHON_MAIN_PREFIX = "/Frameworks/Python.framework/Versions/3.8";
const QString PYTHON_MAIN_BIN_PATH = PYTHON_MAIN_PREFIX + "/bin/python3";
const QString PYTHON_LIB_PATH = "/lib/python3.8";
const QString PYTHON_SITE_PACKAGES = PYTHON_MAIN_PREFIX + PYTHON_LIB_PATH + "/site-packages";
const QStringList PYTHON_SYS_PATHS = QStringList () << "/lib-dynload" << "/site-packages";
#endif