Fix UnlockPartialProofState lockhash attribute

[konradkonrad]

This class was missing the lockhash attribute in order to be used like
any other lock state object.

Fixes #2794

[palango]

Yes, the problem stems from this code:

raiden/raiden/transfer/channel.py

Lines 472 to 493 in 2ea7fef

	lock = channel_state.partner_state.secrethashes_to_lockedlocks.get(secrethash)
	# If the lock was not found in locked locks, this means that we've received
	# the secret for the locked transfer but we haven't unlocked it yet. Lock
	# expiry in this case could still happen which means that we have to make
	# sure that we check for "unclaimed" locks in our check.
	if not lock:
	lock = channel_state.partner_state.secrethashes_to_unlockedlocks.get(secrethash)
	lock_registered_on_chain = (
	secrethash in channel_state.our_state.secrethashes_to_onchain_unlockedlocks
	)
	if not lock:
	msg = (
	f'Invalid LockExpired message. '
	f'Lock with secrethash {pex(secrethash)} is not known.'
	)
	return (False, msg, None)
	current_balance_proof = get_current_balanceproof(sender_state)
	merkletree = compute_merkletree_without(sender_state.merkletree, lock.lockhash)

This is actually reported by mypy:

raiden/transfer/channel.py:479: error: Incompatible types in assignment (expression has type "UnlockPartialProofState", variable has type "HashTimeLockState")