Make unit tests hermetic: serve HTTP fixtures locally instead of lavr.github.io #215

Several unit tests download fixtures from the internet, so CI depends on external sites being available.

On 2026-09-28 the unit jobs failed on all Python versions because the gh-pages branch (which serves http://lavr.github.io/python-emails/tests/... via GitHub Pages) had been deleted on 2026-04-02. The branch was restored and CI is green again, but the dependency remains.

Affected tests

Fixtures from http://lavr.github.io/python-emails/tests/ (the tests/ directory of the gh-pages branch, ~800 KB):

• emails/testsuite/loader/test_loaders.py: test_loaders, test_loaders_with_params, test_loader_image_callback (campaignmonitor-samples/...)
• emails/testsuite/message/test_send.py: test_send_letters (python-logo.gif)
• emails/testsuite/store/test_store.py: test_lazy_http (campaignmonitor-samples/sample-template/...)
• emails/testsuite/test_utils.py: test_url_fix (url-fix/Пушкин А.С.jpg)

Other external hosts:

• emails/testsuite/test_utils.py: test_fetch_url fetches http://google.com.

Proposal

• Move the needed fixtures from gh-pages:tests/ into emails/testsuite/ (some, e.g. oldornament, already exist in emails/testsuite/loader/data).
• Serve them from a local HTTP server fixture (see emails/testsuite/test_url_safety.py for an example of a threaded http.server fixture) and use its base url instead of lavr.github.io.
• Since 1.1.3 the default emails.utils.url_validator rejects loopback addresses, so these tests need a fixture that sets emails.utils.url_validator = None (via monkeypatch).
• Optionally keep a small number of real-network tests behind a marker (e.g. network) excluded from the default unit run.

Expected outcome

pytest -m "not e2e and not django" passes without network access.