[lint] exclude icons.py from more linters

Summary:
This file is generated semi-automatically by running contrib/gen-icons.sh. After updating it with a new icon, I got flake8 errors related to the number of blank lines between blocks of code.
The number of blank lines arount the imports can trigger ISort.

Depends on D14540

Test Plan: Add extra blank lines and other formatting changes to icons.py and check that linters no longer care.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14541