Since python 3.8, "python3-config --libs" doesn't return -lpython3.8. So the macro BOOST_PYTHON fail with the following stack trace:
configure:15049: g++ -o conftest -O3 -std=c++14 -DNDEBUG -I/usr/include/python3.8 -I/usr/include/python3.8 -L/usr/lib -lcrypt -lpthread -ldl -lutil -lm -lm -L/lib64 conftest.o -lboost_python3 -lcrypt -lpthread -ldl -lutil -lm -lm >&5 /usr/bin/ld: conftest.o: in function boost::python::api::slice_nil::~slice_nil()':
conftest.cpp:(.text._ZN5boost6python3api9slice_nilD2Ev[_ZN5boost6python3api9slice_nilD5Ev]+0x15): undefined reference to _Py_Dealloc' /usr/bin/ld: conftest.o: in function _GLOBAL__sub_I_PyInit_empty':
conftest.cpp:(.text.startup+0x13): undefined reference to _Py_NoneStruct' /usr/bin/ld: conftest.cpp:(.text.startup+0x1b): undefined reference to _Py_NoneStruct'
/usr/bin/ld: /usr/lib/libboost_python3.so: undefined reference to PyExc_ValueError' /usr/bin/ld: /usr/lib/libboost_python3.so: undefined reference to PyLong_AsLong'
(…)`

Full config.log