P2PKH output are stakeable

[Gnappuraz]

Describe the bug
// UNIT-E TODO: Restrict to witness programs only once #212 is merged (fixes #48) hasn't been removed hence any P2PKH output is considered stakeable but once we try to sign a just created block we'll fail to ExtractP2WPKHKeys() cause the utxo is not a witness compatible one.

To Reproduce
Create a legacy address, send all the funds there, try to propose.

Expected behavior
We should not be able to stake coins that have a non-witness script.

[castarco]

It seems that Github didn't close this issue automatically, but as far as I can see, it's already fixed.

[castarco]

Woops, it says "closed", nor "merged", reopening just in case.