Fix pycodestyle CI scope for bundled Zcash sources

[Copilot]

The pycodestyle workflow started failing because it linted the bundled src/electrumx/lib/zcash subtree, which carries extensive upstream formatting that does not match this repo's lint policy. This change narrows the CI check to repo-owned code and fixes the remaining style violations in tx.py.

• CI scope

  • Exclude src/electrumx/lib/zcash from the pycodestyle job.
  • Keep the existing lint policy unchanged for the rest of src/.

• Local style cleanup

  • Fix the remaining non-Zcash pycodestyle findings in src/electrumx/lib/tx.py:
    • spacing after not
    • inline comment spacing
    • missing blank line before TxPIVX

• Resulting workflow change

  pycodestyle \
    --exclude=src/electrumx/lib/zcash \
    --max-line-length=140 \
    src