[PyROOT] Don't import `libcppyy` directly in PyROOT

[guitargeek]

The libcppyy is an implementation detail of cppyy that we should not include directly.

This is done to anticipate more refactoring later.

[github-actions[bot]]

Test Results

    11 files      11 suites   2d 3h 53m 50s :stopwatch:  2 631 tests  2 625 :white_check_mark: 0 :zzz: 6 :x: 27 316 runs  27 310 :white_check_mark: 0 :zzz: 6 :x:

For more details on these failures, see this check.

Results for commit 87cad49c.

:recycle: This comment has been updated with latest results.

[guitargeek]

All green now, besides an unrelated failure that @egpbos should learn about

[egpbos]

Thanks!