FWIW, the following method do works for me:

import sys

if sys.platform == 'win32':
    import os
    import winreg
    def get_git_binary_path():
        proc_arch = os.getenv('PROCESSOR_ARCHITECTURE', '').lower()
        proc_arch64 = os.getenv('PROCESSOR_ARCHITEW6432','').lower()

        if proc_arch == 'x86' and not proc_arch64:
            arch_keys = {0}
        elif proc_arch == 'x86' or proc_arch == 'amd64':
            arch_keys = {winreg.KEY_WOW64_32KEY, winreg.KEY_WOW64_64KEY}
        else:
            raise Exception("Unhandled arch: %s" % proc_arch)

        for arch_key in arch_keys:
            for base_key in [winreg.HKEY_LOCAL_MACHINE, winreg.HKEY_CURRENT_USER]:
                try:
                    with winreg.OpenKey(
                            base_key, r"Software\GitForWindows",
                            0, winreg.KEY_READ | arch_key) as key:
                        path = winreg.QueryValueEx(key, 'InstallPath')[0]
                        git_path = os.path.join(path, 'bin', 'git.exe')
                        if os.path.exists(git_path):
                            return git_path
                except FileNotFoundError:
                    continue
        return None
else:
    # on normal platforms, git is just in the PATH
    def get_git_binary_path():
        return 'git'

@tardyp thanks a lot for the suggestion and the research!
If you could provide the code above in a PR I would happily merge it.