Rename tkinter import from Tk to tk.

[anntzer]

Currently we import tkinter as Tk, which is a bit confusing as there
is also a tkinter.Tk class (for example, both tkinter.mainloop --
currently aliased as Tk.mainloop -- and tkinter.Tk.mainloop exist).
Instead, import tkinter as tk; this shorthand is also consistent with
the tkinter docs (https://docs.python.org/3/library/tkinter.html).

Noted while investigating #12703.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[tacaswell]

This has conflicts, presumably from #12707

[anntzer]

rebased