Allow to lock a key

[nir0s]

It would be nice if a user could "lock" a certain key to prevent it from being modified or deleted.

An API like this could be provided:

stash.lock('key')
...

stash.delete('key')
# The key could is locked and therefore could not be deleted
stash.is_locked('key')
# True

stash.unlock('key')
...