Saving and loading expressions with symbolic functions makes print crash

[rburing]

The following innocent code crashes SageMath 9.4 as well as 9.5.beta5:

f = function('f')(x)
save(f, 'f')
f = load('f.sobj')
print(f)

Output:

TypeError                                 Traceback (most recent call last)
~/Downloads/sage-9.5.beta5/local/var/lib/sage/venv-python3.7/lib/python3.7/site-packages/sage/symbolic/pynac_impl.pxi in sage.symbolic.expression.py_print_function_pystring (build/cythonized/sage/symbolic/expression.cpp:10229)()
    499         olist = [func._name]
    500     olist.extend(['(', ', '.join(map(repr, args)), ')'])
--> 501     return ''.join(olist)
    502 
    503 cdef stdstring* py_print_function(unsigned id, args):

TypeError: sequence item 0: expected str instance, bytes found
...
------------------------------------------------------------------------
Unhandled SIGSEGV: A segmentation fault occurred.
This probably occurred because a *compiled* module has a bug
in it and is not properly wrapped with sig_on(), sig_off().
Python will now terminate.
------------------------------------------------------------------------
Segmentation Fault

Originally reported in ​Ask SageMath question #59632.

[mkoeppe]

dup of #30018