refactor(verification): organization and typing improvements [part 1/9]

[glevco]

Motivation

This PR is the first one in a series of 9 PRs that finishes the vertex verification refactors. The objective is to completely remove all inheritance from the verification process, removing implicit dependencies between verifiers, streamlining the verification process and improving clarity on the scope/origin of each verification method, all to improve code readability, maintainability, and patching (for the simulator).

This is done incrementally. In previous PRs, we moved the verification out of the model classes. Now, in this series, we'll first change inheritance to composition, and then completely remove the composition too, leaving each verifier with the least possible amount of dependencies.

Also, in this PR, we install the typing_extensions dev dependency. This is a project from the official Python organization, and what it does is enable use of new type system features on older Python versions. In other words, it simply allows us to use features from the typing module that are not available in all Python versions we support.

Specifically, for this refactor we'll use assert_never(), a function to be used were we would normally use something like assert False, but that can be correctly type-checked by mypy. This improves robustness of match statements for example, that can be made exhaustive with this. We also use the @override annotation in next PRs, improving robustness and safety guarantees of inheritance. We may also find other useful typing features in the future that wouldn't be available otherwise.

Acceptance Criteria

• Change BaseTransaction and all subclasses to use TxVersion as the version type, instead of int.
• Install the typing_extensions dev dependency.
• Move verify_unsigned_skip_pow() from TransactionVerifier to create_tx.py. This resolves Move verify_unsigned_skip_pow() #825.
• In VerificationService:
  • Change all isinstance() checks to type() is, which is more restrictive.
  • Change raise NotImplementedError usages in match statements to assert_never(), which can be checked by the static type checker.
  • Remove validate_vertex_error(), inlining it HathorManager.
• Set self.verifiers variable in test_verification, updating all usages. This will be useful in the next PRs.
• No behavior should be changed by this PR.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (8671304) 85.18% compared to head (c632ffe) 85.19%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #830   +/-   ##
=======================================
  Coverage   85.18%   85.19%           
=======================================
  Files         285      285           
  Lines       22655    22652    -3     
  Branches     3444     3443    -1     
=======================================
- Hits        19299    19298    -1     
- Misses       2671     2673    +2     
+ Partials      685      681    -4     

Files	Coverage Δ
hathor/transaction/base_transaction.py	96.13% <ø> (ø)
hathor/transaction/block.py	94.52% <ø> (ø)
hathor/transaction/merge_mined_block.py	72.97% <ø> (ø)
hathor/transaction/resources/create_tx.py	93.75% <100.00%> (+1.29%)	⬆️
hathor/transaction/token_creation_tx.py	99.11% <ø> (ø)
hathor/transaction/transaction.py	94.76% <ø> (ø)
hathor/verification/transaction_verifier.py	91.54% <ø> (-0.51%)	⬇️
hathor/manager.py	82.21% <50.00%> (+0.02%)	⬆️
hathor/verification/verification_service.py	89.77% <81.25%> (-0.66%)	⬇️

... and 4 files with indirect coverage changes

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