fix pip extension download pattern for PythonPackage easyblock

[smoors]

(created using eb --new-pr)

I suspect that this has changed for more recent versions of pip

I discovered this when setting use_pip_for_deps = True, which could be handy for automatically detecting missing dependencies. pip_sanity_check does not always work for that because it does not report missing dependencies of dependencies.

example pip output:

 Collecting geographiclib<2,>=1.49
  Downloading geographiclib-1.50-py3-none-any.whl (38 kB)

[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