font.Font.__init__, font.families, and font.names have a 'root=None' argument and start with 
    if not root:
        root = tkinter._default_root
But font.nametofont does not, and so it calls Font without passing a root argument:
     return Font(name=name, exists=True)

Font fails if there is no default root.  There cannot be one if, as recommended, one disables it.

import tkinter as tk
from tkinter import font
tk.NoDefaultRoot()
root = tk.Tk()
font.nametofont('TkFixedFont')
# AttributeError: module 'tkinter' has no attribute '_default_root'

Proposed fix: add 'root=None' parameter to nametofont (at end, to not break code) and 'root=root' to Font call.

New changeset 36a779e64c580519550aa6478c5aa8c58b8fa7b6 by Desmond Cheong in branch 'master':
bpo-35728: Add root parameter to tkinter.font.nametofont() (GH-23885)
https://github.com/python/cpython/commit/36a779e64c580519550aa6478c5aa8c58b8fa7b6

Has the PR already solved the problem? If yes, could this issue not be closed?

The code I gave above now exits with a clearer message.
RuntimeError: No master specified and tkinter is configured to not support default root

But now, in 3.10, the master can be specified, so that
  fnt = font.nametofont('TkFixedFont', root)
works.  Thank you Serhiy and desmondcheongzx.