refactor: create non-null hash property

[glevco]

Motivation

Currently, the BaseTransaction.hash property is of type Optional[bytes], which is pretty inconvenient considering the hash has (and should have) a value most times. For that, we have to use asserts in multiple places to guarantee the value is not None.

This PR renames the existing optional property to _hash, creating a new calculated hash property that performs the assert for us. By doing it like this, every place that already uses the existing hash property will continue to work (only a few places that rely on the possibility of the value being None had to be updated).

Acceptance Criteria

• Rename current optional BaseTransaction.hash property to _hash and add new non-null hash property.
• Remove some unnecessary asserts.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 85.15%. Comparing base (a2ee438) to head (6c494e6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #967      +/-   ##
==========================================
+ Coverage   85.05%   85.15%   +0.10%     
==========================================
  Files         294      294              
  Lines       22782    22783       +1     
  Branches     3428     3430       +2     
==========================================
+ Hits        19378    19402      +24     
+ Misses       2718     2702      -16     
+ Partials      686      679       -7     

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