Enable Mypy to check that enum match is exhaustive

[ysangkok]

Fixes #49

[dgpv]

Thanks for the code. I've decided to take it up a notch and make assert_never into a function exported from bitcointx.util. I've also changed some typing code in psbt, and the use of TypeVar allowed to uncover a few wrinkles, like using == instead of is for checking the enums.

My code is at #51

[dgpv]

Using TypeVar there to turn read_psbt_keymap into a generic function allowed to avoid asserting assert isinstance(key_type, PSBT_InKeyType) explicitly

[ysangkok]

Good, that is better