Address PyLint 2.13.7 warnings

PyLint 2.13.7 reports some new warnings for various Python bits in the repository:

• v9.19, v9.18

  ************* Module conftest
  bin/tests/system/doth/conftest.py:34:28: E0601: Using variable 'stderr' before assignment (used-before-assignment)

  This is a blocker, but it is trivial to fix.

• v9.16

  ************* Module setup
  bin/python/setup.py:12:0: W0402: Uses of a deprecated module 'distutils.core' (deprecated-module)

  This is strictly speaking not a blocker because this warning is only raised for Python 3.10+ (see PEP 632) while Debian "bullseye", which we use PyLint on, only has Python 3.9. That being said, we might as well fix this problem today and be done with it. The exact way of addressing the PyLint warning will be discussed in a relevant merge request.

Blocks #3260 (closed)