Add @type_check_only to stub-only private classes in stdlib, round 2

[brianschubert]

New hits from python/mypy#19574:

error: importlib.metadata._EntryPointBase is not present at runtime. Maybe mark it as "@type_check_only"?
error: statistics._SizedIterable is not present at runtime. Maybe mark it as "@type_check_only"?
error: tkinter.ttk._ElementCreateImageKwargs is not present at runtime. Maybe mark it as "@type_check_only"?
error: tkinter.ttk._ElementCreateVsapiKwargsMargin is not present at runtime. Maybe mark it as "@type_check_only"?
error: tkinter.ttk._ElementCreateVsapiKwargsPadding is not present at runtime. Maybe mark it as "@type_check_only"?
error: tkinter.ttk._ElementCreateVsapiKwargsSize is not present at runtime. Maybe mark it as "@type_check_only"?
error: tkinter.ttk._Layout is not present at runtime. Maybe mark it as "@type_check_only"?

Introduced in #14348, #15249, and one missed due to python/mypy@565e42f

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉