Restrict possible return types of `_put` and `_delete` methods

[mlondschien]

I forgot: Do we (want to) restrict the return types of _put and _delete methods implemented by the backends? IIUC they generally return the key at which data was saved (or deleted).

Originally posted by @mlondschien in #25 (comment)

[xhochy]

👍 sounds like we should do that and verify that all backends do that (should be through mypy's checks already then).

[mlondschien]

mypy cannot check as many of the underlying modules are not typed sadly.

[mlondschien]

This

minimalkv/tests/basic_store.py

Line 24 in e682c5c

assert isinstance(key, str)

could test

        new_key = store.put(key, value)
        assert new_key == key

We could do the same for delete, move and copy.

[mlondschien]

delete

Most backends return None, except for

mongo, which returns a pymongo.results.DeleteResult:

minimalkv/minimalkv/db/mongo.py

Lines 32 to 33 in e682c5c

	def _delete(self, key: str) -> str:
	return self.db[self.collection].delete_one({"_id": key})

redis, which returns 1 if a file was deleted and 0 else:

minimalkv/minimalkv/memory/redisstore.py

Lines 27 to 28 in e682c5c

	def _delete(self, key: str) -> int:
	return self.redis.delete(key)