test: disable pytest plugin autoload under tox

[l46983284-cpu]

Summary

test_lazy needs a clean process so it can prove six.moves.html_parser loads html.parser lazily. When pytest auto-loads ambient third-party plugins, some of them can import html.parser (or pull in packages that do) before the test runs, so the precondition fails even though six itself is fine.

This sets PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 in the tox testenv so the suite only uses pytest itself (tox already depends on pytest alone). That matches maintainer feedback: avoid picking up arbitrary installed plugins rather than special-casing every preloaded module.

Also adds my name to CONTRIBUTORS.

Fixes #335

Test plan

• tox -e py / python -m pytest test_six.py under this tox env (plugin autoload off)
• test_lazy no longer depends on a skip for preloaded html.parser
• full test_six.py green in the tox configuration

Note on earlier approach

An earlier revision of this PR skipped test_lazy when html.parser was already imported. That is no longer the product unit; the tox env var is the fix on the current head.

[benjaminp]

Why would Django be imported in the six test suite?

[l46983284-cpu]

It wouldn't. six doesn't import Django.

Issue #335 is about running the suite in an environment where Django (or anything else) has already imported html.parser, so assert html_name not in sys.modules fails before the lazy-load check can run. I dropped the Django wording from the skip; it now only skips when that pre-condition is already broken.

[benjaminp]

Why does django get loaded into six's test process though?

[l46983284-cpu]

It is not six that loads Django.

On issue #335 the pytest banner shows ambient plugins, including hypothesis. When Django is also installed in that environment, importing the hypothesis pytest plugin pulls hypothesis.internal.compat, which does from django.test import TransactionTestCase if Django is importable. That import graph reaches django.utils.html, which imports stdlib html.parser before test_lazy runs.

Controls I ran:

• Django installed, no hypothesis plugin: import django does not load html.parser; test_lazy passes.
• Django + hypothesis plugin entrypoint load: both django and html.parser are already in sys.modules at collection time.
• Direct import django.utils.html is enough to put html.parser in sys.modules.

So the failure mode is polluted site-packages / plugin autoload, not the six test suite importing Django. The skip is only for the broken pre-condition (html.parser already imported). I expanded the comment in the PR to spell that path out.

[benjaminp]

I'm not sure we can be robust about global pollution of the Python environment. I wonder if there's a way to avoid picking up arbitrary and unneeded plugins like this.

[l46983284-cpu]

Good point — the failure comes from pytest auto-loading installed plugins, not from six importing anything extra.

I changed the approach: PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 in tox.ini so the suite only uses pytest itself (tox already depends on pytest alone). That env var has been supported for a long time; older pytest just ignores it, so it stays safe for the old matrix. I did not put --disable-plugin-autoload in setup.cfg/addopts, because that flag is new (8.4+) and would break older pytest.

I also dropped the skip-based workaround and restored the original assert html_name not in sys.modules pre-condition under that cleaner invocation.