Fix test-case gdb.python/py-event-load.exp for target board
remote-gdbserver-on-localhost using gdb_download_shlib.

Tested on x86_64-linux.

This changes skip_shlib_tests to invert the sense, and renames it to
allow_shlib_tests.

This changes skip_python_tests to invert the sense, and renames it to
allow_python_tests.

This changes various tests to use "require" for the Python feature.

This changes some tests to use "require !skip_shlib_tests".

This commit is the result of running the gdb/copyright.py script,
which automated the update of the copyright year range for all
source files managed by the GDB project to be updated to include
year 2023.

Currently, Python code can use event registries to detect when gdb
loads a new objfile, and when gdb clears the objfile list.  However,
there's no way to detect the removal of an objfile, say when the
inferior calls dlclose.

This patch adds a gdb.free_objfile event registry and arranges for an
event to be emitted in this case.