> ctypes.util.find_library("libc") used to work in 3.8, not working in 3.9. As I said before, ctypes.util.find_library("c") works in both 3.8 and 3.9.

no, it doesn't work (and it shouldn't) neither in python 3.8 nor 3.7

    Python 3.8.5 (default, Sep  4 2020, 07:30:14) 
    Type 'copyright', 'credits' or 'license' for more information
    IPython 7.19.0 -- An enhanced Interactive Python. Type '?' for help.

    In [1]: import ctypes.util
    In [2]: a = ctypes.util.find_library("libc")
    In [3]: print(a)
    None

    Python 3.7.6 (default, Jan  8 2020, 19:59:22) 
    Type 'copyright', 'credits' or 'license' for more information
    IPython 7.16.1 -- An enhanced Interactive Python. Type '?' for help.
    Python 3.7.6 (default, Jan  8 2020, 19:59:22) 

    import ctypes.util
    a = ctypes.util.find_library("libc")
    print(a)
    None

as I said, adding prefix "lib" is wrong