ThreadingHTTPServer missing from http.server stub

[stephen-dexda]

Was added to http.server in 3.7: https://docs.python.org/3/library/http.server.html#http.server.ThreadingHTTPServer

Missing from https://github.com/python/typeshed/blob/master/stdlib/3/http/server.pyi

Looks like it got missed from #1965?

[srittau]

Patches welcome!

[Hanaasagi]

If nobody is working on this, I am willing to do.