Add some python type hints for keypair

[camfairchild]

Addresses part of #86

[roman-opentensor]

Suggested change

	from typing import Optional, Final
	from typing import Optional, Final, TYPE_CHECKING
	if TYPE_CHECKING:
	from scalecodec import ScaleBytes

[camfairchild]

will this work without scalecodec in the requirements?

[roman-opentensor]

will this work without scalecodec in the requirements?

kind of yes for the build. because TYPE_CHECKING will only work during the call of functions/methods where the annotation is checked.
SDK has scalecodec in the requirements. It gonna work via SDK.
But (theoretically) if someone wants to install the wallet package independently of SDK or BTCLI and tries to create an instance of the Keypair class, they will get an error when calling the sign and verify methods. I would add scalecodec to the wallet requirements with a version equal to the version in BTCLI and SDK