@eryksun almost idly I ran your ctypes code in the built interpreter. As written, it produces a lower-case c:\\ as yours did.

But...

Running Debug|Win32 interpreter...
Python 3.8.0a0 (heads/master:7a3056f, Jul 23 2018, 08:23:33) [MSC v.1912 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import ctypes, sys
>>>
>>> sys.executable
'c:\\work-in-progress\\python\\cpython\\PCbuild\\win32\\python_d.exe'
>>> kernel32 = ctypes.WinDLL('kernel32', use_last_error=True)
>>> path = (ctypes.c_wchar * 4)()
>>> kernel32.GetVolumePathNameW(sys.executable, path, 4)
1
>>> path.value
'C:\\'
>>>