The sqlite3 module should not be built as a core module

[erlend-aasland]

The sqlite3 extension module accidentally got upped as a core module (Py_BUILD_CORE_MODULE) in gh-105927. We'd like to keep most extension modules away from the internal API, so I'm reverting this specific change.

cc. @vstinner

Linked PRs

• gh-106104: Use public weakref APIs in sqlite3 #106105

[erlend-aasland]

As noted on the issue, I don't plan to follow this up. See #106105 (comment). If we decide to convert the _sqlite3 extension module to use the Limited API in the future, we can revisit this.