Merge #13047: [trivial] Tidy blocktools.py

Summary:
4d355bfb2b [tests] tidy up blocktools.py (John Newbery)
cab8be5adf [tests] Fix flake8 warnings in blocktools.py (John Newbery)
b184127db2 [doc][trivial] no retargeting in regtest mode (Jesse Cohen)

Pull request description:

Tidies up the blocktools.py module:

- fixes flake8 warnings
- changes function-level comments to docstrings.

Takes in @skeees's commit b184127db2f10c70637ce61ba678efcab0f08a88

Tree-SHA512: 0f4c59ac8ccc9057492ec1996381e73380d65e85240f2ba9607174c0743d3a1853c4ed35a9e1bc704b2b6d6d823ac77aa7e81bd150cf5033de79293c24b791b0

Partial backport of Core PR13047

Reviewer note: Changes involving create_transaction are already more up-to-date with Core than what you see in the PR, so these
parts were skipped, among others.

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D6354