FicklingContextManager improvements

[thomas-chauchefoin-tob]

Noticed a few potential issues while working on neighbour code:

• __exit__ only restores pickle.load but hook.run_hook() patched more APIs than that and they won't be restored.
• Could the internal hook clash with a pre-existing ML global hook? Same with __exit__?
• wrapped_load is dead code, so support for max_acceptable_severity likely never worked here.