Respect custom loaders in harden-pyyaml

[drdavella]

Overview

harden-pyyaml should not replace custom loaders

Description

• In order to avoid breaking custom code, we need to be careful to respect the presence of custom YAML loaders
• While we can't detect whether such loaders are actually safe, making this change would likely result in broken code

[codecov]

Codecov Report

Merging #132 (13063c8) into main (954a20e) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #132   +/-   ##
=======================================
  Coverage   95.82%   95.82%           
=======================================
  Files          64       64           
  Lines        2612     2612           
=======================================
  Hits         2503     2503           
  Misses        109      109           

Files	Coverage Δ
src/core_codemods/harden_pyyaml.py	100.00% <ø> (ø)