[3.6] bpo-31459: Rename IDLE's module browser from Class Browser to Module Browser. (GH-3704)

[miss-islington]

The original module-level class and method browser became a module
browser, with the addition of module-level functions, years ago.
Nested classes and functions were added yesterday. For back-
compatibility, the virtual event <>, which
appears on the Keys tab of the Settings dialog, is not changed.
Patch by Cheryl Sabella.
(cherry picked from commit cd99e79)

https://bugs.python.org/issue31459

[the-knights-who-say-ni]

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[miss-islington]

@csabella and @terryjreedy: Backport status check is done, and it's a success ✅ .

[miss-islington]

Thanks, @terryjreedy!