Fix accidental change of the default source_urls in the PythonPackage EB

[Flamefire]

The original list was modified as a side effect, a copy is required here

edit: fix for bug introduced in #2364

[hound]

Some files could not be reviewed due to errors:

Traceback (most recent call last):

Traceback (most recent call last):
  File "/home/linters/.local/bin/flake8", line 5, in 
    from flake8.main.cli import main
ModuleNotFoundError: No module named 'flake8'

[boegel]

lgtm